strerror_r: Avoid clobbering the strerror buffer when possible.

strerror_r: Avoid clobbering the strerror buffer when possible.

* lib/strerror_r.c (strerror_r): Merge the three implementations.
Handle gnulib defined errno values here. When strerror() returns NULL
or an empty string, return EINVAL.
* lib/strerror.c (strerror): Always call strerror_r. Don't handle
gnulib defined errno values here.
* modules/strerror (Depends-on): Add verify, strerror_r-posix.