curve key names as in libcurve, symetry design, a few logic improvements

- Rename the curve keys and makes code symetric as in libcurve
- Correct logic into "case ZMQ_CURVE_SERVER" (now ZMQ_CURVE_NODE) : this sets as_server, and mechanism = ZMQ_CURVE inconditionally.
- Make mandatory the call of zmq_setsockopt onto ZMQ_CURVE_SERVER (now ZMQ_CURVE_NODE)  (was not necessary for the client), preventing a symetric design, and IMHO laxist ws code quality for the application.
- Correct logic into "case ZMQ_PLAIN_SERVER" (now ZMQ_PLAIN_NODE) : this sets as_server, and mechanism = ZMQ_PLAIN inconditionally.
- Make mandatory the call of zmq_setsockopt onto ZMQ_PLAIN_SERVER  (now ZMQ_PLAIN_NODE)  (was not necessary for the client), preventing a symetric design, and IMHO laxist ws code quality for the application.