[M] Change in libosmo-abis[master]: Add CAS channel support

Attention is currently required from: jolly.

laforge has posted comments on this change by jolly. ( https://gerrit.osmocom.org/c/libosmo-abis/+/41140?usp=email )

Change subject: Add CAS channel support ......................................................................

Patch Set 1: Code-Review-1

(1 comment)

Patchset:

PS1: as stated in the osmo-e1d patch, the 4 CAS bits should be exposed on a per-B-channel basis, i.e. the demux of CAS frame should happen inside osmo-e1d (like it is in DAHDI) and then the libosmo-abis interface will also be on that basis. This means that different applications can e.g. open a different subset of B-channels, without a single/central instance managing the CAS timeslot.