AP_HAL_Linux: set thread name for ease debug

Use pthread_setname_np() to set thread name so it's easier to debug
what't going on with each of them. This is the example output of the
relevant par of "ps -Leo class,rtprio,wchan,comm":

FF      12 futex_ ArduCopter.elf
FF      15 usleep sched-timer
FF      14 hrtime sched-uart
FF      13 poll_s sched-rcin
FF      11 hrtime sched-tonealarm
FF      10 hrtime sched-io

libraries/AP_HAL_Linux/Scheduler.cpp

@@ -49,6 +49,10 @@ void LinuxScheduler::_create_realtime_thread(pthread_t *ctx, int rtprio,
         panic(PSTR("Failed to create thread"));
     }
     pthread_attr_destroy(&attr);
+
+    if (name) {
+        pthread_setname_np(*ctx, name);
+    }
 }
 
 void LinuxScheduler::init(void* machtnichts)