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/include/defs.h

@@ -29,6 +29,9 @@
 #define defs_h_defined
 
 #include	<ast.h>
+#if !defined(AST_VERSION) || AST_VERSION < 20111111L
+#error libast version 20111111 or later is required
+#endif
 #if !SHOPT_MULTIBYTE
     /* disable multibyte without need for further '#if SHOPT_MULTIBYTE' */
 #   undef mbwide