libosmo-trx/ep: allow ctrl socket to accept/reply to any peer

By default, the socket stays connect()ed to the configured peer, so
the kernel silently drops datagrams from anyone else.  This is fine
for the normal osmo-bts/osmo-trx/trxcon use case, where both ends of
the link are fixed and known ahead of time.

Add an optional promiscuous mode that enables ttcn3-bts-test to
inject path simulation TRXC commands from its own source port: when
enabled, leave the ctrl socket unconnected and always reply to the
actual sender of the last received CMD (tracked in chan->ctrl_peer),
rather than only the configured peer.

This mode will be used by the upcoming osmo-trx-proxy.

Change-Id: I97075eb350e4270f4e909d493ba692e6b65be750
Related: OS#6672
---
M libosmo-trx/include/osmocom/trx/ep.h
M libosmo-trx/src/trx_ep.c
M tests/libosmo-trx/trx_ep_test.c
M tests/libosmo-trx/trx_ep_test.err
M tests/libosmo-trx/trx_ep_test.ok
5 files changed, 171 insertions(+), 25 deletions(-)