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/DESIGN

@@ -153,12 +153,10 @@ sh directory:
 	28.	timers.c contains code for multiple event timeouts.
 	29.	trestore contains the code for restoring the parse
 		tree from the file created by tdump.
-	30.	userinit.c contains a dummy userinit() function.
-		This is now obsolete with the new version of sh_main().
-	31.	waitevent.c contains the sh_waitnotify function so
+	30.	waitevent.c contains the sh_waitnotify function so
 		that builtins can handle processing events when the
 		shell is waiting for input or for process completion.
-	32.	xec.c is the main shell execution loop.
+	31.	xec.c is the main shell execution loop.
 
 edit directory:
 	1.      completion.c contains code for command and file generation and