Problem: ZMQ background threads are unnamed

Solution: use pthread API to set the name. For now call every thread
"ZMQ b/g thread". Would be nice to number the I/O threads and name
explicitly the reaper thread, but in reality a bit of internal API
churn would be necessary, so perhaps it's not worth it.
This is useful when debugging a process with many threads.

src/ctx.cpp

@@ -412,6 +412,7 @@ void zmq::ctx_t::start_thread (thread_t &thread_, thread_fn *tfn_, void *arg_) c
 {
     thread_.start(tfn_, arg_);
     thread_.setSchedulingParameters(thread_priority, thread_sched_policy);
+    thread_.setThreadName ("ZMQ b/g thread");
 }
 
 void zmq::ctx_t::send_command (uint32_t tid_, const command_t &command_)