bsdinstall: replace ntpdate by ntpd_sync_on_start

• change current NTP services offered by the FreeBSD Installer;
• no longer offer ntpdate to be enabled and started on boot;
• start offering the option to make ntpd set the date and time on boot itself.

The motivation for this change comes from the ntpdate(8) manpage:

Note: The functionality of this program is now available in the ntpd(8)
program. See the -q command line option in the ntpd(8) page. After a
suitable period of mourning, the ntpdate utility is to be retired from
this distribution.

Approved by: cy (src), dteske (src)
Differential Revision: https://reviews.freebsd.org/D36206