Various minor cleanups and fixes

The more notable ones are:

src/lib/libast/features/standards:
- Do not redefine _GNU_SOURCE and _FILE_OFFSET_BITS if already
  defined from $CCFLAGS. Thanks to @hyanias for the heads-up.
  (re: 289dd46c)

src/cmd/ksh93/data/builtins.c,
src/cmd/ksh93/include/shell.h,
src/cmd/ksh93/sh/args.c,
src/cmd/ksh93/sh/name.c:
- Remove -T test code activation option. It was basically unused.
  The only thing it did was intentionally introduce a memory leak
  in table_unset() if the 4th bit in the option argument was set.
  A search in ast-open-history reveals a few more trivial test uses
  that were later deleted, but nothing interesting.

src/cmd/ksh93/tests/{basic,path}.sh:
- Skip a couple of tests on AIX avoid hangs, at least one of which
  is not ksh's fault. Thanks to @HansH111 for the report.

src/cmd/ksh93/tests/builtins.sh:
- Change one awk use to a more portable sed invocation to placate
  systems with ancient awk commands, such as AIX. (re: de795e1f)

src/lib/libast/features/standards

@@ -155,8 +155,12 @@ elif tst note{ GNU (glibc) or Cygwin }end compile{
 			return 0;
 		}
 	}end {
+		#ifndef _GNU_SOURCE
 		#define _GNU_SOURCE	1
+		#endif
+		#ifndef _FILE_OFFSET_BITS
 		#define _FILE_OFFSET_BITS 64
+		#endif
 		#define basename	basename	/* avoid string.h defining this in conflict with AST basename(3) */
 	}
 elif tst note{ _ALL_SOURCE & _POSIX_SOURCE & _POSIX_C_SOURCE & _XOPEN_SOURCE & __EXTENSIONS__ works }end compile{

src/lib/libast/sfio/sfhdr.h

@@ -433,7 +433,7 @@
 
 #define SECOND		1000	/* millisecond units */
 
-/* macros do determine stream types from 'struct stat' data */
+/* macros to determine stream types from 'struct stat' data */
 #ifndef S_IFDIR
 #define S_IFDIR	0
 #endif