Add two new tunables / sysctls to controll reboot after panic:

kern.poweroff_on_panic which, when enabled, instructs a system to
power off on a panic instead of a reboot.

kern.powercyle_on_panic which, when enabled, instructs a system to
power cycle, if possible, on a panic instead of a reboot.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D13042