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/cmd/ksh93/include/shell.h

@@ -377,7 +377,6 @@ struct Shell_s
 	Dt_t		*typedict;
 	Dt_t		*inpool;
 	char		ifstable[256];
-	unsigned long	test;
 	Shopt_t		offoptions;	/* options that were explicitly disabled by the user on the command line */
 	Shopt_t		glob_options;
 	Namval_t	*typeinit;