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
Approved by: gibbs (mentor)
Approved by: re (gjb)
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