Automatically run ntpd as non-root when possible.

Ntpd needs only a subset of full root privileges to do its job. Specifically
it needs the ability to manipulate system time, and to re-bind to a
privileged UDP port after interface changes. The mac_ntpd(4) policy module
(see r336525) can grant these privs.

These changes detect the availability of mac_ntpd(4). If enabled, and if the
ntpd configuration is fairly vanilla, it automatically runs ntpd as the
non-root user 'ntpd' (uid 123). "Vanilla" means the config doesn't include
command line or ntp.conf options changing the location of files or using any
files/dirs likely to be inaccessible to user ntpd. Ntpd can still run as
non-root when using such options, but the admin must ensure all required
files and dirs are accessible, and then set ntpd_user=ntpd in rc.conf.

Note that these changes also address PR 199127 by using the command_args
technique suggested in the patch. They also tangentially address PR 113552,
which is primarily about inconsistent filenames in documentation, but some
of the inconsistancy was caused by old code in rc.d/ntpd which is leftover
from the intial import from netbsd. There was code to do chroot setup which
required the use of the netbsd clockctl(4) device; that code never had any
effect on freebsd, because we lack that device and don't build ntpd with the
options that would allow using it.

PR: 113552 199127
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D16050