Add support for E1/T1 based BTSs to OsmoBSC
Contrary to OsmoNITB, OsmoBSC never was able to talk to E1/T1 based BTSs so far.
See #2547 for handling TRAU frames in osmo-mgw to get them converted to/from RTP for AoIP.
This ticket is more about the signaling side. It's TBD on the details how the signaling is handled. We could possibly open signaling slots directly from the BSC (via libosmo-abis/e1_input), or we could have osmo-mgw open those slots and convert the HDLC payload / LAPD into something that can be transported over IP to osmo-bsc.
Once this has been implemented + tested, we have a fully-functional replacement for OsmoNITB, even for legacy E1/T1 BTSs.