strerror_r: fix OpenBSD behavior on 0

strerror_r: fix OpenBSD behavior on 0

OpenBSD treats strerror_r(0,,) as a success, but with a message
"Undefined error: 0"; while this is distinct from strerror_r(-1,,)
returning "Unknown error: -1", it does not imply success.  Meanwhile,
if buf is short enough for ERANGE, then we can't use strstr to look
for "Unknown" or "Undefined" in the resulting message, like we had
been doing for strerror().  Fix this by shifting the burden - now
the strerror-override code guarantees that 0 will have an
override when needed.

* lib/strerror-override.c (strerror_override): Also override 0
when needed.
* lib/strerror-override.h (strerror_override): Likewise.
* lib/strerror.c (strerror): Simplify, now that 0 override is done
earlier.
* lib/strerror_r.c (strerror_r): Likewise.
* m4/strerror.m4 (gl_FUNC_STRERROR): Split detection of 0
behavior...
(gl_FUNC_STRERROR_0): ...into new macro.
* m4/strerror_r.m4 (gl_FUNC_STRERROR_R): Replace strerror_r if 0
is overridden.
(gl_FUNC_STRERROR_R_WORKS): Avoid extra tests if 0 is broken.
* modules/strerror-override (Files): Add strerror.m4.
(configure.ac): Also provide override for 0 when needed.
* doc/posix-functions/strerror.texi (strerror): Document this.
* doc/posix-functions/perror.texi (perror): Likewise.

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