Handle timeout/EINTR gracefully for ZmqPoll

[markuspf]

I just pushed a patch to ZeroMQInterface that adjusts ZmqPoll's behaviour to just return if the process gets a EINTR signal. This happens for example if the process that called ZmqPoll has a forked child process who forks a child itself (and this was exposed in https://github.com/gap-packages/JupyterKernel).

Currently we just return an empty list of results, making it impossible to decide whether we timed out, or got EINTR. This should be handled better.