Knowledge of the magic 127.127.t.u address convention for refclocks is
now confined to ntp_config.c and used only for interpreting
configuration syntax.

In theory we could now assign a refclock any unique IP address we
like; neither ntpd nor ntpq would care any more. In practice it's
probably best to continue assigning only 127.127.t.u addresses so
that (at least for a while) we refrain from confusing legacy ntpq
versions.

Together with the elimination of refnumtoa(), this sets the stage
for entirely removing the external visibility of numeric driver
types, and a better clock-configuration syntax.