Allow building on AIX

This makes ksh build at least on AIX 7.1 on RISC (PowerPC).

There are 4 regression test failures:

        leaks.sh[159]: memory leak on PATH reset before PATH search
        (leaked approx 220 KiB after 16384 iterations)

        pty.sh[351]: POSIX sh 104(C): line 364: expected
        "^done\r?\n$", got EOF

        signal.sh[280]: subshell ignoring signal does not send
        signal to parent (expected 'SIGUSR1', got 'done')

        signal.sh[282]: parent does not wait for child to complete
        before handling signal

src/cmd/INIT/iffe.sh:
- Unset LIBPATH on AIX. The features/pty output{ ... }end will fail
  to link to libiconv otherwise, causing a build failure. See:
  https://www.ibm.com/support/pages/member-libiconvso2-not-found-archive

src/cmd/builtin/pty.c:
- CMIN is not defined on AIX, so set it to 1 if it's not defined.

src/cmd/ksh93/README:
- Update list of tested OSs.

src/cmd/INIT/iffe.sh

@@ -26,6 +26,9 @@
 # NOTE: .exe a.out suffix and [\\/] in path patterns for dos/nt
 
 (command set -o posix) 2>/dev/null && set -o posix
+case `uname -s` in
+AIX)	unset LIBPATH ;;
+esac
 
 command=iffe
 version=2012-07-17 # update in USAGE too #