Add rawmemchr module, matching glibc.

Add rawmemchr module, matching glibc.

* modules/string (Makefile.am): New indicator.
* m4/string_h.m4 (gl_HEADER_STRING_H_DEFAULTS): Set it.
* lib/string.in.h (rawmemchr): Declare when appropriate.
* modules/rawmemchr: New file.
* m4/rawmemchr.m4: Likewise.
* lib/rawmemchr.c: Likewise.
* modules/rawmemchr-tests: Likewise.
* tests/test-rawmemchr.c: Likewise.
* doc/glibc-functions/rawmemchr.texi (rawmemchr): Document
module.
* modules/strchrnul (Depends-on): Add rawmemchr.
* lib/strchrnul.c (strchrnul): Optimize a corner case.

Signed-off-by: Eric Blake <ebb9@byu.net>