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/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