libast/regex: do not use small block size for stack (re: e311f1e)

Opening the match stack with the STK_SMALL flag causes the stk code
to allocate memory in blocks of 64*sizeof(char*) instead of
1024*sizeof(char*). This caused a significant slowdown which was
exposed by the extglob.ksh module of shbench. Thanks to @JohnoKing
for noticing and reporting the problem.

src/lib/libast/regex/regcomp.c: regcomp():
- Remove STK_SMALL from the stkopen() option bit flags.

Resolves: ksh93/ksh#440

src/lib/libast/regex/regcomp.c

@@ -3278,7 +3278,7 @@ regcomp(regex_t* p, const char* pattern, regflags_t flags)
 	if (!(p->env = (Env_t*)alloc(disc, 0, sizeof(Env_t))))
 		return fatal(disc, REG_ESPACE, pattern);
 	memset(p->env, 0, sizeof(*p->env));
-	if (!(p->env->mst = stkopen(STK_SMALL|STK_NULL)))
+	if (!(p->env->mst = stkopen(STK_NULL)))
 		return fatal(disc, REG_ESPACE, pattern);
 	memset(&env, 0, sizeof(env));
 	env.regex = p;