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>