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.

lib/.gitignore

@@ -109,9 +109,7 @@
 /mbchar.c
 /mbchar.h
 /mbrtowc.c
-/mbschr.c
 /mbsinit.c
-/mbsrchr.c
 /mbswidth.c
 /mbswidth.h
 /mbuiter.h