kill explicit ZMQ::Socket->close, simplify cleanups

	modified:   lib/POEx/ZMQ/Socket.pm
Constants; export POSIX-only consts

	modified:   lib/POEx/ZMQ/Constants.pm
	modified:   tools/zmq_constants.pl
ZMQ::Socket cleanups, POD outline

	modified:   lib/POEx/ZMQ/Socket.pm