Remove obsolete docs, keeping official description

The lib/package/ast-open.* are almost entirely obsolete after
most of the AST utilities were removed to concentrate on ksh93
development. The only part that is both relevant and not repeated
elsewhere is AT&T's introductory blurb on ksh93.

README.md:
- Add the official AT&T ksh93 description from ast-open.html,
  reformatted to Markdown with a couple of added hyperlinks.

lib/package/ast-open.README,
lib/package/ast-open.html:
- Removed. (Note that the historic ksh93 changelog in these files
  is also included in src/cmd/ksh93/RELEASE).

README.md

@@ -77,3 +77,107 @@ Start by reading the information printed by:
 ```sh
 ./bin/shtests --man
 ```
+
+## What is ksh93?
+
+The following is the official AT&T description from 1993 that came with the
+ast-open distribution. The text is original, but hyperlinks were added here.
+
+----
+
+KSH-93 is the most recent version of the KornShell Language described in
+"The KornShell Command and Programming Language," by Morris Bolsky and David
+Korn of AT&T Bell Laboratories, ISBN 0-13-182700-6. The KornShell is a shell
+programming language, which is upward compatible with "sh" (the Bourne
+Shell), and is intended to conform to the IEEE P1003.2/ISO 9945.2
+[Shell and Utilities standard](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/contents.html).
+KSH-93 provides an enhanced programming environment in addition to the major
+command-entry features of the BSD shell "csh". With KSH-93, medium-sized
+programming tasks can be performed at shell-level without a significant loss
+in performance. In addition, "sh" scripts can be run on KSH-93 without
+modification.
+
+The code should conform to the
+[IEEE POSIX 1003.1 standard](http://www.opengroup.org/austin/papers/posix_faq.html)
+and to the proposed ANSI-C standard so that it should be portable to all
+such systems. Like the previous version, KSH-88, it is designed to accept
+eight bit character sets transparently, thereby making it internationally
+compatible. It can support multi-byte characters sets with some
+characteristics of the character set given at run time.
+
+KSH-93 provides the following features, many of which were also inherent in
+KSH-88:
+
+* Enhanced Command Re-entry Capability: The KSH-93 history function records
+  commands entered at any shell level and stores them, up to a
+  user-specified limit, even after you log off. This allows you to re-enter
+  long commands with a few keystrokes - even those commands you entered
+  yesterday. The history file allows for eight bit characters in commands
+  and supports essentially unlimited size histories.
+* In-line Editing: In "sh", the only way to fix mistyped commands is to
+  backspace or retype the line. KSH-93 allows you to edit a command line
+  using a choice of EMACS-TC or "vi" functions. You can use the in-line
+  editors to complete filenames as you type them. You may also use this
+  editing feature when entering command lines from your history file. A user
+  can capture keystrokes and rebind keys to customize the editing interface.
+* Extended I/O Capabilities: KSH-93 provides several I/O capabilities not
+  available in "sh", including the ability to:
+  * specify a file descriptor for input and output
+  * start up and run co-processes
+  * produce a prompt at the terminal before a read
+  * easily format and interpret responses to a menu
+  * echo lines exactly as output without escape processing
+  * format output using printf formats.
+  * read and echo lines ending in "\\". 
+* Improved performance: KSH-93 executes many scripts faster than the System
+  V Bourne shell. A major reason for this is that many of the standard
+  utilities are built-in. To reduce the time to initiate a command, KSH-93
+  allows commands to be added as built-ins at run time on systems that
+  support dynamic loading such as System V Release 4.
+* Arithmetic: KSH-93 allows you to do integer arithmetic in any base from
+  two to sixty-four. You can also do double precision floating point
+  arithmetic. Almost the complete set of C language operators are available
+  with the same syntax and precedence. Arithmetic expressions can be used to
+  as an argument expansion or as a separate command. In addition there is an
+  arithmetic for command that works like the for statement in C.
+* Arrays: KSH-93 supports both indexed and associative arrays. The subscript
+  for an indexed array is an arithmetic expression, whereas, the subscript
+  for an associative array is a string.
+* Shell Functions and Aliases: Two mechanisms - functions and aliases - can
+  be used to assign a user-selected identifier to an existing command or
+  shell script. Functions allow local variables and provide scoping for
+  exception handling. Functions can be searched for and loaded on first
+  reference the way scripts are.
+* Substring Capabilities: KSH-93 allows you to create a substring of any
+  given string either by specifying the starting offset and length, or by
+  stripping off leading or trailing substrings during parameter
+  substitution. You can also specify attributes, such as upper and lower
+  case, field width, and justification to shell variables.
+* More pattern matching capabilities: KSH-93 allows you to specify extended
+  regular expressions for file and string matches.
+* KSH-93 uses a hierarchal name space for variables. Compound variables can
+  be defined and variables can be passed by reference. In addition, each
+  variable can have one or more disciplines associated with it to intercept
+  assignments and references.
+* Improved debugging: KSH-93 can generate line numbers on execution traces.
+  Also, I/O redirections are now traced. There is a DEBUG trap that gets
+  evaluated before each command so that errors can be localized.
+* Job Control: On systems that support job control, including System V
+  Release 4, KSH-93 provides a job-control mechanism almost identical to
+  that of the BSD "csh", version 4.1. This feature allows you to stop and
+  restart programs, and to move programs between the foreground and the
+  background.
+* Added security: KSH-93 can execute scripts which do not have read
+  permission and scripts which have the setuid and/or setgid set when
+  invoked by name, rather than as an argument to the shell. It is possible
+  to log or control the execution of setuid and/or setgid scripts. The
+  noclobber option prevents you from accidentally erasing a file by
+  redirecting to an existing file.
+* KSH-93 can be extended by adding built-in commands at run time. In
+  addition, KSH-93 can be used as a library that can be embedded into an
+  application to allow scripting.
+
+Documentation for KSH-93 consists of an "Introduction to KSH-93",
+"Compatibility with the Bourne Shell" and a manual page and a README file.
+In addition, the "New KornShell Command and Programming Language" book is
+available from Prentice Hall.