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/files.c

@@ -207,7 +207,7 @@ file_name_split (const char *file_name,
   *base = last_component (file_name);
 
   /* Look for the extension, i.e., look for the last dot. */
-  *ext = mbsrchr (*base, '.');
+  *ext = strrchr (*base, '.');
   *tab = NULL;
 
   /* If there is an extension, check if there is a `.tab' part right