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.

TODO

@@ -11,7 +11,7 @@ Fix regression test failures:
 ______
 Fix build system:
 
-- ksh does not currently build on AIX or QNX.
+- ksh does not currently build on QNX.
 
 ______
 Fix currently known bugs affecting shell scripting. These are identified by

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 #

src/cmd/builtin/pty.c

@@ -116,6 +116,10 @@ USAGE_LICENSE
 #define MODE_666	(S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH)
 #define MAXNAME		64
 
+#ifndef CMIN
+#define CMIN		1
+#endif
+
 #if !_lib_openpty && !_lib__getpty && !defined(_pty_clone)
 # if !_lib_grantpt || !_lib_unlock
 #   if !_lib_ptsname

src/cmd/ksh93/README

@@ -167,22 +167,24 @@ things amiss), two asterisks signify moderate regression test failures
 (some functionality does not work), and three asterisks signify serious
 failures (crashes, and/or essential functionality does not work).
 
-*	DragonFly BSD on x86_64
+*	AIX 7.1 on RISC (PowerPC)
-	FreeBSD on x86_64
+*	DragonFly BSD 5.8 on x86_64
-	GNU/Linux: CentOS on x86_64
+	FreeBSD 12.2 on x86_64
-	GNU/Linux: Debian on x86_64
+	FreeBSD 12.2 on arm64 (thanks to hyenias for donating access to a Pi)
-	GNU/Linux: Gentoo on i386
+	GNU/Linux: CentOS 8.2 on x86_64
-	GNU/Linux: NixOS on x86_64
+	GNU/Linux: Debian 10.7 on x86_64
-	GNU/Linux: Slackware on x86_64
+	GNU/Linux: Gentoo 2.7 on i386
-	GNU/Linux: Ubuntu on x86_64
+	GNU/Linux: NixOS 19.09 on x86_64
-***	HP-UX 11 on pa-risc
+	GNU/Linux: Slackware 14.2 on x86_64
+	GNU/Linux: Ubuntu 16.04 on x86_64
+***	HP-UX B.11.11 on pa-risc
 *	illumos: OmniOS 2020-08-19 (gcc) on x86_64
 	macOS 10.14.6 (Mojave) on x86_64
-***	NetBSD on x86_64
+***	NetBSD 8.1 and 9.0 on x86_64
-**	OpenBSD on x86_64
+*	OpenBSD 6.8 on x86_64
 *	Solaris 11.4 (gcc) on x86_64
 	Solaris 11.4 (Solaris Studio 12.5 cc) on x86_64
-***	UnixWare 7.1.4 on x86
+*	UnixWare 7.1.4 on x86
 ***	Windows 7 using Cygwin on x86
 
 Good luck!!