[ftmod_libpri] Use FTDM_SPAN_USE_PROCEED_STATE and rework state handling.

- Use the newly introduced FTDM_SPAN_USE_PROCEED_STATE flag and FTDM_CHANNEL_STATE_PROCEED.

- Update ftmod_libpri's state machine table (taken from ftmod_sangoma_isdn).

- Move pri_destroycall() to HANGUP_COMPLETE state.

- Try to get a little bit closer to the ISDN states by using pri_acknowledge() in RINGING,
  pri_progress() in PROGRESS and pri_proceeding() in PROCEED state.

- Go to PROGRESS_MEDIA in on_progress() only if there is inband indication available, go to
  PROGRESS if not.

- Go to RINGING state in on_ringing() and remove PROGRESS_MEDIA check.

NOTE: One libpri error message and a VETO (PROGRESS -> RINGING) warning to investigate,
      but changes seem to work fine nonetheless.

Tested-by: SparFux (#freeswitch / #freeswitch-de  @ irc.freenode.net)
Signed-off-by: Stefan Knoblich <s.knoblich@axsentis.de>