libast: regex: backport robustness improvements from 93v- beta

There are two main changes:

1. The regex code now creates and uses its own stack (env->mst)
   instead of using the shared standard stack (stkstd). That seems
   likely to be a good thing.

2. Missing mbinit() calls were inserted. The 93v- code uses a
   completely different multibyte characters API, so these needed
   to be translated back to the older API. But, as mbinit() is no
   longer a no-op as of 300cd199, these calls do stop things from
   breaking if a previous operation is interrupted mid-character.

I think there might be a couple of off-by-one errors fixed as well,
as there are two instances of this change:

-		while ((index += skip[buf[index]]) < mid);
+		while (index < mid)
+			index += skip[buf[index]];

src/lib/libast/regex/regerror.c

@@ -26,7 +26,7 @@
  * POSIX regex error message handler
  */
 
-static const char id[] = "\n@(#)$Id: regex (AT&T Research) 2012-05-31 $\0\n";
+static const char id[] = "\n@(#)$Id: regex (AT&T Research) 2012-09-27 $\0\n";
 
 #include "reglib.h"