This patch adds support to set up a CBSP link so that osmo-cbc connects
as a TCP client to the BSC, which runs as a TCP server.
Similary, support to set up a SBc-AP link so that osmo-cbc connects
as a SCTP client to the MME, which runs as SCTP server.
A new "mode (client|server|disabled)" VTY command is added to use one
mother or the other, and also to disable the link and hence the peer
until it is set again. This is useful if the peer is under manintenance
for instance.
client sockets are created automatically when the "peer" vty node is
exited if the link is not yet created, hence creating it at startup or
if moved back from "disabled" mode. If client socket dies, it will
keep attempting reconnect every 5 seconds.
Related: OS#4945
Change-Id: I3ec54b615b41b56f7a9c64298e3fcaac37f4b60e