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

src/cmd/ksh93/include/fault.h

@@ -98,11 +98,7 @@ struct checkpt
 	int		topfd;
 	int		mode;
 	struct openlist	*olist;
-#if (ERROR_VERSION >= 20030214L)
 	Error_context_t err;
-#else
-	struct errorcontext err;
-#endif
 };
 
 #define sh_pushcontext(shp,bp,n)( (bp)->mode=(n) , (bp)->olist=0,  \

src/cmd/ksh93/include/jobs.h

@@ -118,14 +118,9 @@ extern struct jobs job;
 #ifdef vmlocked
 #define vmbusy()	vmlocked(Vmregion)
 #else
-#if VMALLOC_VERSION >= 20070911L
 #define vmbusy()	(vmstat(0,0)!=0)
 #endif
 #endif
-#endif
-#ifndef vmbusy
-#define vmbusy()	0
-#endif
 
 #define job_lock()	(job.in_critical++)
 #define job_unlock()	\

src/cmd/ksh93/include/streval.h

@@ -29,9 +29,7 @@
 #include	<ast.h>
 #include	<math.h>
 #include	"defs.h"
-#if _AST_VERSION >= 20030127L
-#   include	<ast_float.h>
-#endif
+#include	<ast_float.h>
 
 #if _ast_fltmax_double
 #define LDBL_LLONG_MAX		DBL_LLONG_MAX