strerror_r: avoid corrupting errno on Solaris

strerror_r: avoid corrupting errno on Solaris

On Solaris, a non-zero return was also reflected into errno.
Leaving errno unchanged is a useful feature worth guaranteeing.

* m4/strerror_r.m4 (gl_FUNC_STRERROR_R): Check for Solaris behavior.
* doc/posix-functions/strerror_r.texi (strerror_r): Document it.

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