Changes

libosmo-trx/client: add TRXC client (command queue) API

Add the osmo_trxc_client module: a transport-agnostic generalization
of the TRXC command handling logic in osmo-bts-trx (trx_if.c):

* command queue with a single command in flight;
* retransmission on response timeout (default: 2 s);
* suppression of consecutive duplicate commands;
* RSP<->CMD matching, optionally including the parameters;
* filtering of duplicate responses caused by retransmissions;
* escalation of NACKed critical commands (OSMO_TRXC_F_CRITICAL)
  to the fatal_error call-back;
* TRXD PDU version negotiation (SETFORMAT), incl. the fallback to
  version 0 for old transceivers rejecting it with 'RSP ERR 1'.

The client neither opens nor owns a socket: the application transmits
serialized messages in the tx_msg call-back and feeds received
datagrams into osmo_trxc_client_rx().  The response call-back may
return N > 0 to request a re-transmission of the same command after
N seconds (e.g. POWERON retry, as implemented in osmo-bts-trx).

Change-Id: I817e394f74a10e3adae4a0b58342c82acdf0794e