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.

Change-Id: Ib90a032b38c69ae26023e726992d3f5f7e502fcf
---
M libosmo-trx/include/osmocom/trx/trxc_client.h
M libosmo-trx/src/trxc_client.c
M tests/libosmo-trx/trxc_client_test.c
M tests/libosmo-trx/trxc_client_test.err
M tests/libosmo-trx/trxc_client_test.ok
5 files changed, 118 insertions(+), 5 deletions(-)