libc: Improve POSIX conformance of dirfd()

POSIX states that dirfd() should set errno to EINVAL and return -1 if
dirp does not refer to a valid directory stream. Our interpretation is
that this applies if dirp is null or the file descriptor associated
with it is negative.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55025