proccontrol(1): implement 'nonewprivs'

This adds the 'nonewprivs' mode, corresponding to newly added
procctl(2) commands PROC_NO_NEW_PRIVS_CTL and PROC_NO_NEW_PRIVS_STATUS.

Reviewed By: kib
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D30940