Add ROUTER socket option to introduce self(send an empty message) to new peers, to allow ROUTER<->ROUTER auto-discovery problem.

Not sure that zmq::router_t::identify_peer() - it is the best place for the code to automatically send <NULL> message to new peers.

Anyway it works, and now ROUTER will send <NULL> message to any newly connected peer.
This way ROUTER on other side immediately will know the identity of the remote side.

P.S.
I believe that 'ZMQ_ROUTER_ANNOUNCE_SELF' option name could/should be adjusted.