Disable suspend when we're shutting down. This solves the "tell FreeBSD
to shut down; close laptop lid" scenario which otherwise tended to end
with a laptop overheating or the battery dying.

The implementation uses a new sysctl, kern.suspend_blocked; init(8) sets
this while rc.suspend runs, and the ACPI sleep code ignores requests while
the sysctl is set.

Discussed on: freebsd-acpi (35 emails)
MFC after: 1 week