acl_equiv_mode_np: zero mode on just inited ACL

You can't return 0 and not write the mode if mode_p is non-NULL. That
violates the API contract and in common usage leaves stack trash in
*mode_p.

The acl_equiv_mode_test test passed by accident.

Reviewed by: kevans, markj
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D43278

libc: Purge unneeded cdefs.h

These sys/cdefs.h are not needed. Purge them. They are mostly left-over
from the $FreeBSD$ removal. A few in libc are still required for macros
that cdefs.h defines. Keep those.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D42385

Remove $FreeBSD$: one-line .c pattern

Remove /^[\s*]*__FBSDID\("\$FreeBSD\$"\);?\s*\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

libc/posix1e: Add acl_equiv_mode_np() function.

Reviewed by: kib, debdrup, gbe
Approved by: kib
Differential Revision: https://reviews.freebsd.org/D28255