use enum retired_fd instead of -1

using -1 causes a warning on Windows platform because SOCKET is unsigned.
Updated news for #775
Merge pull request #43 from hintjens/master

Backporting fixes from master
Fixed setting TCP keepalive parameters on Windows
Updated NEWS
Merge pull request #44 from hintjens/master

Issue 797
Fix LIBZMQ-576 - Revert "Merge pull request #510 from miniway/master"

This reverts commit f27eb67e1abb0484c41050e454404cce30647b63, reversing
changes made to a3ae0d4c16c892a4e6c96d626a7c8b7068450336.

Conflicts:
	src/stream_engine.cpp

Conflicts were around additional defaults to the constructor after the
'terminating' default.  The additional defaults were left alone, and
the 'terminating' default was removed.
Merge pull request #46 from vortechs2000/fix-libzmq-576

Fix LIBZMQ-576 - Revert "Merge pull request #510 from miniway/master"
Adjust NEWS to include LIBZMQ-576
Merge pull request #47 from vortechs2000/fix_news

Adjust NEWS to include LIBZMQ-576
Corrects ypipe initialization when conflate is NOT enabled.  Fixes LIBZMQ-584
Update README.md
Update README.md
Merge pull request #51 from EricMCornelius/master

Corrects ypipe initialization when conflate is NOT enabled.
Clarified ZMQ_IDENTITY doc for duplicate identities
Updated NEWS for #584
Fixed #868
Merge pull request #55 from hintjens/master

Fixed issue #868
Fix timestamp counter on s390/s390x

Fixes #811
Only export zmq_* symbols