MFC r220376: Allow strerror(0) and strerror_r(0, ...).

MFC r220376: Allow strerror(0) and strerror_r(0, ...).

Of course, strerror_r() may still fail with ERANGE.

Although the POSIX specification said this could fail with EINVAL and
doing this likely indicates invalid use of errno, most other
implementations permitted it, various POSIX testsuites require it to
work (matching the older sys_errlist array) and apparently some
applications depend on it.

PR:		standards/151316

git-svn-id: svn://svn.freebsd.org/base/stable/8@220664 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f