Of course people still "can" distributed the sources under the
LGPLv3. However we provide COPYING.LESSER with additional grants.
Solution: specify these grants in the header of each source file.
* 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.