Fix unlink(1) for files starting with -

Fix unlink(1) for files starting with -

Restore the original behavior of unlink(1), passing the provided filename
directly to unlink(2), handling the first argument being "--" correctly.

This fixes "unlink -foo", broken in r97533.

PR:		228448
Submitted by:	Brennan Vincent <brennan@umanwizard.com> (original version)
Submitted by:	Yuri Pankov
Reported by:	Brennan Vincent <brennan@umanwizard.com>
Reviewed by:	emaste, kevans, vangyzen, 0mp
Approved by:	re (delphij)
Differential Revision: https://reviews.freebsd.org/D17132