ftmod_libpri: Rework handling of peer-initiated hangup events

ftmod_libpri: Rework handling of peer-initiated hangup events

Use peerhangup flag variable to track whether a hangup has been
initiated by the peer or libpri itself (e.g. Layer 2 timeouts).

These changes fix a couple of problems with hangup events not being
handled properly in some situations:

  - Call abort caused by incoming RESTART on a channel in use
  - T309 timeout after L2 loss
  - Improved hangup handling in libpri-side on_hangup() event handler
    and state_advance() (FreeTDM side)

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>