Replace ntpdate_enable with ntpd_sync_on_start in bsdinstall

[0mp]

Summary

This patch replaces ntpdate_enable with ntpd_sync_on_start in bsdinstall.

The reason behind this change is that the ntpdate daemon is being retired
[1][2] by the NTP upstream. Since even our own manuals [3] suggest to use
ntpd_sync_on_start instead of ntpdate_enable I thought that we may consider
switching to the preferred alternative.

[1]: http://support.ntp.org/bin/view/Dev/DeprecatingNtpdate
[2]: http://man.freebsd.org/ntpdate
[3]: https://www.freebsd.org/cgi/man.cgi?rc.conf

[manu]

Did you read the whole thread that started my commit that added ntpdate_enable ?
https://lists.freebsd.org/pipermail/svn-src-all/2017-November/154088.html

[0mp]

In D16538#352302, @manu wrote:

Did you read the whole thread that started my commit that added ntpdate_enable ?
https://lists.freebsd.org/pipermail/svn-src-all/2017-November/154088.html

No, I only read the commit logs.

Now that I read the whole thread I understand the subtlety of this issue. Thank you, @manu.