ftmod_libpri: First part of the BRI PTMP channel handling changes.

I really need to dig deeper here, some libpri events never fire for
incoming calls and i'll have to find out how mod_freetdm or the
FreeSWITCH core change states on the channel...

Anyway, incoming and outgoing calls still work for me (BRI PTMP TE),
so commit this now and let a wider audience do some more testing.
Signed-off-by: Stefan Knoblich <s.knoblich@axsentis.de>
Tested-by: Stefan Knoblich <s.knoblich@axsentis.de>