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 multi frame. This multiframe
carries 30 individual CAS signaling channels.

Whenever a CAS frame is received, the included 30 CAS channels are
forwarded to the application with a message of 30 octets, one octet for
each CAS channel.

The application requests to transmit a CAS frame by sending a message
with 30 octets, one for each CAS channel. Most recent CAS frame will be
transmitted repeatedly until the application sends a new message.

The 30 octets in the message correspond to the signaling channels of
these 30 time slots:
     1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15
    17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31

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: '0000ABCD'

Change-Id: Ib4f5e6ef02c9b0d1eec2a86d9c48376112805972
---
M include/osmocom/e1d/proto.h
M src/ctl.c
M src/e1d.h
M src/mux_demux.c
M src/proto.c
M src/vty.c
6 files changed, 178 insertions(+), 0 deletions(-)