CommonLibs: DEBUG: Mark interthread classes which are not used in OsmoTRX.
CommonLibs: Allow NULLs to be retrieved from InterthreadQueue.

We need a way to stop InterthreadQueue blocking read to be able to shutdown
a thread. The easiest way to do that is to push NULL to the queue, but the
original implementation will just ignore that and continue blocking. After
the change the blocking read() will exit with NULL result which is perfectly
fine with us.

Ideally we should change all methods of InterthreadQueue to return a status
value to indicate normal exits, timeouts, etc. Right now the only way to
indicate an error is returning NULL, which could be a valid operation.
CommonLibs: Add shutdown() method to the DatagramSocket class.

This method is useful to abort a blocking operations on a socket during shutdown.
CommonLibs: Signal::wait() should return the value which pthread_cond_timedwait() returns.
CommonLibs: Rewrite Threads class to be predictable during start/stop.

Current implementation strictly synchronize thread startup and shutdown
to make sure a thread is actually started on startThread() exit and
that it's stopped on stopThread() exit (or an error is returned).

I also changed the Thread class to be used as a parent class for
an actual thread. This way we could provide much better logical
separation between variables used by different threads. Which will
(hopefully) reduce number of issues with inter-thread communications.
Transceiver52M: Port all threads to the new Thread interface.

Also note that we introduced shutdown() method for Transceiver threads which
implement proper shutdown of threads when they are in blocking read state.
This involves using shutdown() on sockets and pushing NULL to queues.

With this change we should be able to start/stop transceiver channels at
arbitrary moments.
Transceiver52M: Delay is no longer needed at the transceiver exit, threads should shut down cleanly.
CommonLibs: Port InterthreadTest and SocketsTest to the new Thread API.
CommonLibs: Adding a new ThreadsTest testsuite.

It's very basic at this moment. We should add a stress-test for thread
start/stop at least.