Add Channel-Associated Signalling (CAS) support

CAS frames are sent and received repeatedly. They consist of 16 frames
(octets) on time slot 16. This is a CAS multiframe. This multiframe
carries 30 individual CAS signaling channels.

Whenever a CAS frames changes, an event (E1DP_EVT_CAS) is forwarded to
the e1d client. The event includes one octet with the CAS information.

The e1d client requests to transmit a new CAS frame by sending a command
(E1DP_CMD_CAS). The command includes one octet with the CAS information.

The e1d client requests to receive current CAS frame of a given time
slot by setting a query flag in the E1DP_CMD_CAS command. This is useful
if the CAS signals changed before the application was started.

The lower 4 bits of each octet in the message represent the signaling
sub-channels: A, B, C and D. They are packed like this: 'xxxxABCD'

To enable cas support on an interface, set line attribute "cas" via VTY.

Change-Id: Ib4f5e6ef02c9b0d1eec2a86d9c48376112805972
---
M include/osmocom/e1d/proto.h
M include/osmocom/e1d/proto_clnt.h
M src/ctl.c
M src/e1d.h
M src/intf_line.c
M src/mux_demux.c
M src/proto.c
M src/proto_clnt.c
M src/vty.c
9 files changed, 268 insertions(+), 0 deletions(-)