libosmo-trx/client: add optional limit for retransmissions
The client used to retransmit an unacknowledged command forever, like osmo-bts-trx does. trxcon, however, gives up after 3 retransmission attempts, concluding that the transceiver is offline. Add a new API function osmo_trxc_client_set_max_retrans() enabling this behavior: once a command has been retransmitted the given number of times without a response, the client escalates to the fatal_error call-back (with rsp == NULL) and freezes the command queue. The default (0) retains the old behavior (no limit).
This is a preparation for porting trxcon to libosmo-trx.
