strerror_r-posix: fix on MacOS

strerror_r-posix: fix on MacOS

MacOS X 10.5 strerror(0) is "Unknown error: 0", which is not distinguished
from "Unknown error: -1" for out-of-range.  Worse, strerror_r(0,,)
is "Undefined error: 0", although strerror_r for all other out-of-range
values matches strerror.

* m4/strerror.m4 (gl_FUNC_STRERROR): Flush out MacOS bug.
* m4/strerror_r.m4 (gl_FUNC_STRERROR_R_WORKS): Likewise, and fix
logic bug.
* lib/strerror_r.c (strerror_r): Fix the bug.
* lib/strerror.c (strerror): Likewise.
* doc/posix-functions/strerror_r.texi (strerror_r): Document the
problem.
* doc/posix-functions/strerror.texi (strerror): Likewise.
* doc/posix-functions/perror.texi (perror): Likewise.
* tests/test-strerror.c (main): Enhance test.
* tests/test-strerror_r.c (main): Likewise.

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