strerror_r: avoid clobbering strerror on cygwin

strerror_r: avoid clobbering strerror on cygwin

Avoid cygwin strerror_r, since it clobbers strerror buffer through
cygwin 1.7.9.  __xpg_strerror_r is okay, but if a program is
compiled on cygwin 1.7.8 or earlier, it is not available.

* lib/strerror_r.c (strerror_r): Don't use cygwin's strerror_r;
fall back instead to sys_errlist.
* modules/strerror (configure.ac): Add witness.
* tests/test-strerror_r.c (main): Enhance test.
* doc/posix-functions/strerror_r.texi (strerror_r): Document it.
* tests/test-perror2.c (main): Free memory before exit.

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