Add TrustedBSD MAC(4) support to ntpd.

These changes add support for running ntpd as non-root, and improve support
for the --jaildir (chroot) option when running on freebsd. These correspond
to the changes made in the base system with r336525.

The new patches in this change are exactly what was submitted upstream in
https://bugs.ntp.org/show_bug.cgi?id=3509

Approved by: cy@
Differential Revision: https://reviews.freebsd.org/D16396