strerror_r: Avoid clobbering the strerror buffer when possible.

strerror_r: Avoid clobbering the strerror buffer when possible.

* lib/strerror.c: Define _NETBSD_SOURCE. Include <nl_types.h>.
(sys_nerr, sys_errlist): New declarations.
(strerror_r): Be careful not to clobber the strerror buffer on NetBSD,
HP-UX, native Win32, IRIX, and 32-bit Solaris.
* m4/strerror_r.m4 (gl_PREREQ_STRERROR_R): Test whether catgets exists.