release/rc.local: Provide option to shutdown after installation complete

This can be useful instead of reboot if installing in a virtual machine,
and the user wants to modify the VM hardware or virtual media mounts
prior to booting into the newly installed system.

Reported by: Juan Manuel Palacios (@jmp_imaginarium on Twitter)
Approved by: philip
Differential Revision: https://reviews.freebsd.org/D36560