Introducing a poweroff(8) command

[Giorgos Keramidas]

Hi guys,

In response to PR misc/70476 about `halt -p' I wrote a simple patch to
introduce a "poweroff" command that will default to "halt -p" behavior
and inhibit the need for changing the default behavior of halt(8).

o	Does this look ok to you all?

o	Should I suggest using it as a workaround of the behavior
	described in the PR?

The attached patch also fixes a bug described in PR bin/70536 (which I
have submitted after looking around at the source code of reboot.c) but
I can make these two separate diffs if we think it's worth it.

=== References ===
http://www.FreeBSD.org/cgi/query-pr.cgi?pr=misc/70476
http://www.FreeBSD.org/cgi/query-pr.cgi?pr=bin/70536