make fork-safety check optional

[minrk]

allows other code to take over fork-safety, as is planned in ipython/ipykernel#85

[Carreau]

unfortunately need rebase.

[minrk]

rebased

[minrk]

Perhaps what makes more sense is a per-socket fork-safe signaling mechanism (e.g. getattr(socket, '_fork_safe', False)). Or we could assume objects that aren't zmq.Socket are fork-safe. I'm not sure what the best solution is.

[Carreau]

Perhaps what makes more sense is a per-socket fork-safe signaling mechanism (e.g. getattr(socket, '_fork_safe', False)). Or we could assume objects that aren't zmq.Socket are fork-safe. I'm not sure what the best solution is.

Let's go with this one for now.