memchr: detect broken x86_64 and alpha implementations

memchr: detect broken x86_64 and alpha implementations

* modules/memchr-tests (Depends-on): Move mmap detection...
* modules/memchr (Depends-on): ...here.
(configure.ac): Set indicator.
* lib/string.in.h (memchr): Declare replacement.
* modules/string (Makefile.am): Trigger replacement.
* m4/string_h.m4 (gl_HEADER_STRING_H_DEFAULTS): Likewise.
* m4/memchr.m4 (gl_FUNC_MEMCHR): Use mmap to detect platform
bugs.
* doc/posix-functions/memchr.texi (memchr): Document the bug.
* modules/getpagesize (License): Relax license.

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