After installing chrony, we need to update the selection to match
the initial setting in the chrony.conf file.

If no ntp service is installed, display the default "pool.ntp.org".

drakclock can install chrony, so the ntpd and ntp_conf strings
provided by the imezone module should be dynamic, not static.

drakclock only supports selecting a server pool, not individual servers,
so use the "pool" directive instead if the "server" directive. Rename
the subroutines in timezone.pm for clarity.

ntpsec overrides /etc/ntp.conf with settings found in /etc/ntp.d/, so
we need to read/write /etc/ntp.d/use-pool.

Both chrony and ntpsec support the iburst option, so we can write that
unconditionally.