reboot: Fix halt -p behavior

The RB_HALT bit is always set when invoked as halt, so to maintain a
distinction between halt and halt -p, we must check the RB_POWEROFF
bit first.

PR: 291814
Fixes: 4453ec5b8716 ("reboot: Default to a clean shutdown")
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D54320