numfmt: support multi-byte --delimiter

* bootstrap.conf: Depend on mbsstr() to robustly search for a
multi-byte delimiter character (string) within a multi-byte string.
* src/numfmt.c (main): Accept a valid multi-byte delimiter character.
(next_field): Adjust delimiter search from single byte
to multi-byte aware.  Use mbsstr to find the first match.
* tests/misc/numfmt.pl: Add test case.
* NEWS: Mention the improvement.

bootstrap.conf

@@ -176,6 +176,7 @@ gnulib_modules="
   mbs_endswith
   mbschr
   mbslen
+  mbsstr
   mbswidth
   mbszero
   mcel-prefer