maint: don't use mbsr?chr.

Basically, revert ba60c39547a445dee3e07920931b4d7a81843868's move to
mbs* functions, which was prompted by -DGNULIB_POSIXCHECK.  See
<http://lists.gnu.org/archive/html/bison-patches/2012-05/msg00052.html>
and following.

* bootstrap.conf: No longer ask for them.
* src/files.c, src/getargs.c, src/location.c,
* src/parse-gram.c, src/parse-gram.y, src/scan-gram.l,
* src/symtab.c: s/mbs(r?chr)/str$1/g.

src/parse-gram.c

@@ -2431,7 +2431,7 @@ yyreduce:
 #line 343 "src/parse-gram.y"
     {
       char const *skeleton_user = (yyvsp[0].chars);
-      if (mbschr (skeleton_user, '/'))
+      if (strchr (skeleton_user, '/'))
         {
           size_t dir_length = strlen (current_file);
           char *skeleton_build;