Add shutdown/poweroff support to rescue(8)

shutdown is a safer way to power off than reboot (in general), because of
the added shutdown process that it executes via /etc/rc.shutdown . It was
odd that it was missing from rescue(8) since reboot and friends were
added in past commits.

While here, alias poweroff to shutdown for parity with sbin/shutdown/Makefile

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon