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.

IceWM is automatically installed as a fallback DE. So we want to default
to one of the DEs the user actually chose.