MFC r355100:

Allow opt-out of automatic ntpd leapfile checking/fetching.

When a system has no internet connection, or when it is configured to obtain
ntpd leapfiles from some source other than the internet, or even when the
sysadmin has decided for some reason to customize ntp.conf to eliminate use
of the leapfile, the rc.d/ntpd script emits various error messages related
to the file.

This change allows setting the rc var ntp_db_leapfile to NONE to disable all
automatic processing related to that file in rc.d/ntpd.

Differential Revision: https://reviews.freebsd.org/D22461