Copied files to subdirectory v3/zmq/
First conversion to ZeroMQ version 3 only:
* Files merged.
* New import path: github.com/alecthomas/gozmq/v3/zmq
* Package name changed from gozmq to zmq.
* Pkg-config.
* Version check.
* Poll(): fixed timeout argument, now of type time.Duration.
* Fixed type of socket options.
* Removed deprecated stuff.