Hang in QFbVtHandler when application has its own signal handling

There is a hang in QFbVtHandler::handleSignal() if the signal was already handled by the application, since the call to read is blocking (and you can only read the signalfd_siginfo structure if the signal hasn't already been handled). As far as I can see simply making the read non-blocking should fix it.

Attached simple test case that reproduces the hang (easily seen if you run it under strace).