strerror: enforce POSIX ruling on strerror(0)

strerror: enforce POSIX ruling on strerror(0)

http://austingroupbugs.net/view.php?id=382 requires that strerror(0)
succeed, but FreeBSD reports "Unknown error: 0" and fails with EINVAL.

* m4/strerror.m4 (gl_FUNC_STRERROR_SEPARATE): Expose BSD bug.
* m4/strerror_r.m4 (gl_FUNC_STRERROR_R): Likewise.
* lib/strerror_r.c (rpl_strerror_r): Work around it.
* doc/posix-functions/strerror.texi (strerror): Document it.
* doc/posix-functions/strerror_r.texi (strerror_r): Likewise.
* tests/test-strerror.c (main): Strengthen test.
* tests/test-strerror_r.c (main): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>