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/getargs.c

@@ -559,7 +559,7 @@ getargs (int argc, char *argv[])
       case 'F': /* -FNAME[=VALUE]. */
         {
           char* name = optarg;
-          char* value = mbschr (optarg, '=');
+          char* value = strchr (optarg, '=');
           if (value)
             *value++ = 0;
           muscle_percent_define_insert (name, command_line_location (),