reboot: Allow this to be installed as nextboot

Allow nextboot to be a symlink link to reboot. It does everything reboot
does, except doesn't actually setup the sytem to reboot and reboot. Also,
don't accept the reboot args related to rebooting when in nextboot mode.

Sponsored by: Netflix
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D43830