These were exposed to users, but have subsequently been removed as
sockopts. They are currently only being used by ZAP, so I've moved it to
a simpl function call (actually it's only used in one case even in that,
so there may be a further simplification possible there).

Looks like linger is honoured properly, but shutting down the session
causes the pipe termination to come from that side - because the local
pipe then shuts down right away it seems to trigger a terminated on the
other end instead of waiting. This way we trigger the termination from
the local end and then terminate the session.

Add zmq_send_const for constant data

Fix compile error on windows.

Rename ZMQ_REQ_SEND_RESETS -> ZMQ_REQ_STRICT.

It defaults to enabled. Switch it off to be able to send a new message
before the previous reply has been received.

Add ZMQ_REQ_SEND_RESETS option.

Added note to clarify policy for embedded apps

* Fixes a terminate() call on a dangling pointer in the SEND_RESETS
  case.
* Fixes recv_reply_pipe() never receiving a message once the pipe it is
  waiting on is terminated.

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.

Add ZMQ_REQ_REQUEST_IDS option.

* Documentation:
The default behavior of REQ sockets is to rely on the ordering of messages
to match requests and responses and that is usually sufficient. When this option
is set to 1, the REQ socket will prefix outgoing messages with an extra frame
containing a request id. That means the full message is (request id, 0,
user frames...). The REQ socket will discard all incoming messages that don't
begin with these two frames.

* Behavior change: When a REQ socket gets an invalid reply, it used to
  discard the message and return EAGAIN. REQ sockets still discard
  invalid messages, but keep looking at the next one automatically
  until a good one is found or there are no more messages.
* Add test_req_request_ids.

Zap updates

The functon returns string representation of peer's
IP address. We will need this to update ZAP implementation.

Last step in test_spec_req is still failing; disabled it.

Add static libraries to CMake, minor housekeeping.

README file moved to GitHub format and extension, drop from packaging and default to NEWS file for popup.

Fix intermittent failure in test_spec_req.

There still is a problem when a stale peer's message
arrives at the REQ just after a request was sent to that peer.

Fix coding style in spec tests.

REQ sockets drop replies from unasked peers.

* Add lb_t::sendpipe() that returns the pipe that was used for sending,
  similar to fq_t::recvpipe().
* Add forwarder functions to dealer_t to access these two.
* Add logic to req_t to ignore replies on pipes that are not the one
  where the request was sent.
* Enable test in test_spec_req.

Converted README to markdown

update Makefile to include new files

Problem: new spec test cases don't all work

* disabled the specific tests that do not work (yet) on libzmq
* cleaned up one source (test_spec_rep.c) but the others need similar work
* added sleep in test_spec_rep to allow connects time to happen; this would
  not be needed if we connected out to the REP peers instead in from them,
  but I didn't want to change the logic of the test code.