Hunt for a free channel for incoming calls that do not
preselect a channel (pevent->ring.channel == -1).

Verify the preselected channel for calls that do specify a channel
and in case the channel is already taken, hunt for a free one,
or abort with an error message (if the preselection was exclusive).

TE-mode channel selection is the same as before
(there's still room for improvement, though, but i'll save that for later).

The MSN/DDI filter code is moved into the TE-mode section (only useful there).

The duplicate ring detection had to be reworked. We now store the
call reference (CRV) in caller_data->call_reference of the selected channel
and do a CRV -> channel look up with find_channel_by_cref()
at the top of on_ring().

NOTE: This is only lightly tested (NT/TE mode), i'd either have to
      write a lot of custom code to check it toroughly or the need for
      a scriptable ISDN stack...
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>