Support application metadata through ZMQ_METADATA

Lets the application set per-connection metadata.
Metadata is specified as "X-key:value" and set using zmq_setsockopt, eg:
zmq_setsockopt (s, ZMQ_METADATA, "X-key:value", 11);

The peer can then obtain the metadata from a received message:
char *data = zmq_msg_gets(msg, "X-key");

include/zmq.h

@@ -585,6 +585,7 @@ ZMQ_EXPORT void zmq_threadclose (void *thread);
 #define ZMQ_BINDTODEVICE 92
 #define ZMQ_ZAP_ENFORCE_DOMAIN 93
 #define ZMQ_LOOPBACK_FASTPATH 94
+#define ZMQ_METADATA 95
 
 /*  DRAFT 0MQ socket events and monitoring                                    */
 /*  Unspecified system errors during handshake. Event value is an errno.      */