On XenServer the "halt" message is used instead of "poweroff", which
makes FreeBSD halt but not poweroff (as expected when issuing a
shutdown from the VM manager). Fix this by using the same handler
for both "halt" and "poweroff".

NB: The "halt" signal seems to be used on XenServer only.  The OSS
    Xen toolstack (xl) uses "poweroff" instead.

Submitted by:	Roger Pau Monné
Sponsored by:	Citrix Systems R&D
Reviewed by:	gibbs
MFC after:	2 days