Add DONTWAIT flag for send command

[izludec]

Add new flag to send function, which allow zmq send message without freezing thread if you choose PAIR mode.

[enwi]

@izludec Did you test my proposed change?

[izludec]

It works for me(without errors) wirh two flags true, but I wasnt test that ZMQ_SNDMORE works properly. Not sure that zmq allow to use it both

[enwi]

In theory ZeroMQ allows a number of flags being passed to zmq_send, but they need to be or'd (option | option) first.

[enwi]

So the question is, should we add flags for each and every option or do it like ZeroMQ does it and just pass an integer?