Background thread names (#2784)

* Add ZMQ_THREAD_NAME_PREFIX ctx option

include/zmq.h

@@ -613,6 +613,7 @@ ZMQ_EXPORT void zmq_threadclose (void* thread);
 #define ZMQ_MSG_T_SIZE 6
 #define ZMQ_THREAD_AFFINITY 7
 #define ZMQ_THREAD_AFFINITY_DFLT -1
+#define ZMQ_THREAD_NAME_PREFIX 8
 
 /*  DRAFT Socket methods.                                                     */
 ZMQ_EXPORT int zmq_join (void *s, const char *group);