Remove SHOPT_BASH; keep &> redir operator, '-o posix' option

On 16 June there was a call for volunteers to fix the bash
compatibility mode; it has never successfully compiled in 93u+.
Since no one showed up, it is now removed due to lack of interest.

A couple of things are kept, which are now globally enabled:

1. The &>file redirection shorthand (for >file 2>&1). As a matter
   of fact, ksh93 already supported this natively, but only while
   running rc/profile/login scripts, and it issued a warning. This
   makse it globally available and removes the warning, bringing
   ksh93 in line with mksh, bash and zsh.

2. The '-o posix' standard compliance option. It is now enabled on
   startup if ksh is invoked as 'sh' or if the POSIXLY_CORRECT
   variable exists in the environment. To begin with, it disables
   the aforementioned &> redirection shorthand. Further compliance
   tweaks will be added in subsequent commits. The differences will
   be fairly minimal as ksh93 is mostly compliant already.

In all changed files, code was removed that was compiled (more
precisely, failed to compile/link) if the SHOPT_BASH preprocessor
identifier was defined. Below are other changes worth mentioning:

src/cmd/ksh93/sh/bash.c,
src/cmd/ksh93/data/bash_pre_rc.sh:
- Removed.

src/cmd/ksh93/data/lexstates.c,
src/cmd/ksh93/include/shlex.h,
src/cmd/ksh93/sh/lex.c:
- Globally enable &> redirection operator if SH_POSIX not active.
- Remove warning that was issued when &> was used in rc scripts.

src/cmd/ksh93/data/options.c,
src/cmd/ksh93/include/defs.h,
src/cmd/ksh93/sh/args.c:
- Keep SH_POSIX option (-o posix).
- Replace SH_TYPE_BASH shell type by SH_TYPE_POSIX.

src/cmd/ksh93/sh/init.c:
- sh_type(): Return SH_TYPE_POSIX shell type if ksh was invoked
  as sh (or rsh, restricted sh).
- sh_init(): Enable posix option if the SH_TYPE_POSIX shell type
  was detected, or if the CONFORMANCE ast config variable was set
  to "standard" (which libast sets on init if POSIXLY_CORRECT
  exists in the environment).

src/cmd/ksh93/tests/options.sh,
src/cmd/ksh93/tests/io.sh:
- Replace regression tests for &> and move to io.sh. Since &> is
  now for general use, no longer test in an rc script, and don't
  check that a warning is issued.

Closes: #9
Progresses: #20

src/cmd/ksh93/sh.1

@@ -3409,7 +3409,7 @@ successfully rename it to
 otherwise, delete the temporary file.
 .BI >; word
 cannot be used with the
-.IR exec (2).
+.IR exec
 built-in.
 .TP
 .BI >> word
@@ -3433,7 +3433,7 @@ except that if the command completes successfully,
 is truncated to the offset at command completion.
 .BI <>; word
 cannot be used with the
-.IR exec (2).
+.IR exec
 built-in.
 .TP
 \f3<<\fP\*(OK\f3\-\fP\*(CK\f2word\fP
@@ -3492,20 +3492,26 @@ The standard input is duplicated from file descriptor
 .I digit
 (see
 .IR dup (2)).
-Similarly for the standard output using
-\f3>&\^\f2digit\fR.
+.TP
+.BI >& digit
+The standard output is duplicated from file descriptor
+.IR digit .
 .TP
 .BI <& digit \-
 The file descriptor given by
 .I digit
 is moved to standard input.
-Similarly for the standard output using
-\f3>&\^\f2digit\f3\-\fR.
+.TP
+.BI >& digit \-
+The file descriptor given by
+.I digit
+is moved to standard output.
 .TP
 .B <&\-
 The standard input is closed.
-Similarly for the standard output using
-.BR >&\- .
+.TP
+.B >&\-
+The standard output is closed.
 .TP
 .B <&p
 The input from the co-process is moved to standard input.
@@ -3593,6 +3599,11 @@ for reading and store
 the file descriptor number in variable
 .BR n .
 .PP
+A special shorthand redirection operator \fB&>\fR\fIword\fR is available; it
+is equivalent to \fB>\fR\fIword\fR\fB 2>&1\fR. It cannot be preceded by any
+digit or variable name. This shorthand is disabled if the \fIposix\fR shell
+option is active.
+.PP
 The order in which redirections are specified is significant.
 The shell evaluates each redirection in terms of the
 .RI ( "file descriptor" ", " file )
@@ -7020,6 +7031,11 @@ of the pipeline have completed, and the return value
 will be the value of the last non-zero command
 to fail or zero if no command has failed.
 .TP 8
+.B posix
+Enable POSIX standard compatibility mode. This option
+is on by default if ksh is invoked as \fBsh\fR. It
+disables the \fB&>\fR redirection shorthand.
+.TP 8
 .B privileged
 Same as
 .BR \-p .