AuxTask classes require proper cleanup

[giuliomoro]

When the destructor is called, it should stop the thread and then (wrap_)pthread_join before it starts destroying anything.

For NonRt, this could be achieved with either:

• a "fake" __wrap_sendto() just in order to unblock the thread so that it can break the loop
• using timed select() before read() in the non-rt thread so that every so often it checks for the condition and breaks the loop ( see e.g.: core/Midi.cpp)

The former approach is probably better - if it works - as it would save the timeout-ed wakeups due to using select(), thought this is not critical (we are talking timeouts in the 50-100ms range, so no huge CPU load).

There is some commented-out code in AuxTaskNonRt::cleanup() that could be used as a starting point, though it was causing problems.