ftmod_libpri: go to PROGRESS_MEDIA in on_ringing() if there is inband information available and rework channel opening to not skip state changes when the channel is already open.

The latter part affects on_ringing(), on_progress() and on_proceeding().

Somewhat experimental and untested (apart from a compile run).
Signed-off-by: Stefan Knoblich <s.knoblich@axsentis.de>