ftmod_libpri: Major cleanup, enhance configuration checks

- Check D and B-channel availability (could be extended to check B-channel count)

- Drop usage of spri->private_info, use spri->span instead
- Use accessor functions where possible
- Rename ftdmchan to chan where possible
- Various other cleanups
Signed-off-by: Stefan Knoblich <s.knoblich@axsentis.de>
Tested-by: Stefan Knoblich <s.knoblich@axsentis.de>