procctl(2): add PROC_NO_NEW_PRIVS_CTL, PROC_NO_NEW_PRIVS_STATUS

This introduces a new, per-process flag, "NO_NEW_PRIVS", which
is inherited, preserved on exec, and cannot be cleared. The flag,
when set, makes subsequent execs ignore any SUID and SGID bits,
instead executing those binaries as if they not set.

The main purpose of the flag is implementation of Linux
PROC_SET_NO_NEW_PRIVS prctl(2), and possibly also unpriviledged
chroot.

Reviewed By: kib
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D30939