Remove legacy code for older libast versions

Since ksh 93u+m comes bundled with libast 20111111, there's no need
to support older versions, so this is another cleanup opportunity.

src/cmd/ksh93/include/defs.h:
- Throw an #error if AST_VERSION is undefined or < 20111111.
  (Note that _AST_VERSION is the same as AST_VERSION, but the
  latter is newer and preferred; see src/lib/libast/features/api)

All other changed files:
- Remove legacy code for versions older than the currently used
  versions, which are:
  _AST_VERSION    20111111
  ERROR_VERSION   20100309
  GLOB_VERSION    20060717
  OPT_VERSION     20070319
  SFIO_VERSION    20090915
  VMALLOC_VERSION 20110808

src/cmd/ksh93/sh/fault.c

@@ -45,7 +45,7 @@ static int	cursig = -1;
 #if !_std_malloc
 #   include	<vmalloc.h>
 #endif
-#if  defined(VMFL) && (VMALLOC_VERSION>=20031205L)
+#if  defined(VMFL)
     /*
      * This exception handler is called after vmalloc() unlocks the region
      */
@@ -154,7 +154,7 @@ void	sh_fault(register int sig)
 			shp->trapnote |= SH_SIGSET;
 			if(sig <= shp->gd->sigmax)
 				shp->sigflag[sig] |= SH_SIGSET;
-#if  defined(VMFL) && (VMALLOC_VERSION>=20031205L)
+#if  defined(VMFL)
 			if(abortsig(sig))
 			{
 				/* abort inside malloc, process when malloc returns */