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/sh/args.c

@@ -187,12 +187,6 @@ int sh_argopts(int argc,register char *argv[])
 	 	    case 'D':
 			on_option(&newflags,SH_NOEXEC);
 			goto skip;
-		    case 'T':
-			if (opt_info.num)
-				sh.test |= opt_info.num;
-			else
-				sh.test = 0;
-		    	continue;
 		    case 's':
 			if(setflag)
 			{

src/cmd/ksh93/sh/name.c

@@ -2435,8 +2435,6 @@ static void table_unset(register Dt_t *root, int flags, Dt_t *oroot)
 					Sfdouble_t d = nv_getnum(nq);
 					nv_putval(nq,(char*)&d,NV_LDOUBLE);
 				}
-				else if(sh.test&4)
-					nv_putval(nq, sh_strdup(nv_getval(nq)), NV_RDONLY);
 				else
 					nv_putval(nq, nv_getval(nq), NV_RDONLY);
 				sh.subshell = subshell;