src/cmd/INIT/package.sh: add install subcommand

This now allows easy installation via:
bin/package install DESTINATION_DIRECTORY [ COMMAND ... ]

Commands are installed in bin and share/man subdirectories.
If no command is specified, ksh and shcomp are assumed.

src/cmd/ksh93/README

@@ -198,6 +198,18 @@ character locales and really want to save some memory and CPU cycles, add
 '-DAST_NOMULTIBYTE' to CCFLAGS to compile out all multibyte character
 handling in ksh and supporting libraries. Not recommended for most users.
 
+To install, use: bin/package install DESTINATION_DIRECTORY [ COMMAND ... ]
+
+Any command from the arch directory can be installed. If no COMMAND is
+specified, ksh and shcomp are assumed.
+
+The DESTINATION_DIRECTORY is created if it does not exist. Commands are
+installed in its bin subdirectory and each command's manual page, if
+available, is installed in share/man.
+
+Destination directories with whitespace or shell pattern characters in their
+pathnames are not yet supported.
+
 For more information, run:
 
 	bin/package help
@@ -205,13 +217,6 @@ For more information, run:
 Many other commands in this repo self-document via the --help, --man and
 --html options; those that do have no separate manual page.
 
-Automated installation is not supported yet. To install manually:
-
-	cp arch/$(bin/package host type)/bin/ksh /usr/local/bin/
-	cp src/cmd/ksh93/sh.1 /usr/local/share/man/man1/ksh.1
-
-(adapting the destination directories as required).
-
 The build should also generate shcomp, a program that will precompile
 a script.  ksh93 is able to recognize files in this format and process
 them as scripts.  You can use shcomp to send out scripts when you