- used msleep (10) in most places instead of zmq_sleep (1)
- may cause failures on slower machines
- to change, modify SETTLE_TIME in testutil.h
- tested down to 1 msec on fast boxes

* ZMQ_REQ_STRICT was negative option (default 1) which goes against
  the standard, where defaults are zero. I renamed this to
  ZMQ_REQ_RELAXED.

* ZMQ_REQ_REQUEST_IDS felt clumsy and describes the technical solution
  rather than the problem/requirement. I changed to ZMQ_REQ_CORRELATE
  which seems more explicit.

* Removed redundant Z85 code and include files from project
* Simplified use of headers in test cases (now they all just use testutil.hpp)
* Export zmq_z85_encode() and zmq_z85_decode() in API
* Added man pages for these two functions

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

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.