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

This reverts commit f27eb67e1a, reversing
changes made to a3ae0d4c16.

https://zeromq.jira.com/browse/LIBZMQ-576

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.

src/stream_engine.hpp

@@ -162,7 +162,6 @@ namespace zmq
         std::string endpoint;
 
         bool plugged;
-        bool terminating;
 
         int (stream_engine_t::*read_msg) (msg_t *msg_);