ntpd: Use the ntpd -u option in preference to the rc su plumbing

Using the rc plumbing to setuid(2) is preferred as it allows the user
to use the -i option in ntpd_flags to chroot ntpd.

Chrooting ntpd by default will be a 2025 project.

MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D48191