PR228448: `unlink -foo` , the original use of `unlink`, no longer works.

[yuripv]

Summary

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

This fixes the unlink -foo, broken in r97533.

Reported by: Brennan Vincent <[e-mail]>
Submitted by: Brennan Vincent <[e-mail]>
PR: 228448

Test Plan

$ kyua test -k /usr/tests/Kyuafile bin/rm
bin/rm/rm_test:unlink_dash_filename -> passed [0.088s]

Results file id is usr_tests.20180912-125312-617400
Results saved to /home/yuri/.kyua/store/results.usr_tests.20180912-125312-617400.db

1/1 passed (0 failed)

[emaste]

I'll send the request to re@

[0mp]

The manpage update looks fine.