Merge pull request #620 from ckamm/req-id

Add ZMQ_REQ_REQUEST_IDS option.
Add ZMQ_REQ_SEND_RESETS option.

This allows making a new request on a REQ socket by sending a new
message. Without the option set, calling send() after the first message
is done will continue to return an EFSM error.

It's useful for when a REQ is not getting a response. Previously that
meant creating a new socket or switching to DEALER.