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/subshell.c

@@ -72,11 +72,7 @@ static struct subshell
 	struct Link	*svar;	/* save shell variable table */
 	Dt_t		*sfun;	/* function scope for subshell */
 	Pathcomp_t	*pathlist; /* for PATH variable */
-#if (ERROR_VERSION >= 20030214L)
 	struct Error_context_s *errcontext;
-#else
-	struct errorcontext *errcontext;
-#endif
 	Shopt_t		options;/* save shell options */
 	pid_t		subpid;	/* child process id */
 	Sfio_t*	saveout;/*saved standard output */