Improve shutdown signal safeness

The signal handler no longer acquires the service list lock which removes
a number of deadlock possibilities from the shutdown process. Instead, a
global shutdown flag is set that serves the same purpose as the individual
service shutdown flags did.

include/maxscale/service.h

@@ -171,6 +171,9 @@ typedef enum count_spec_t
 #define SERVICE_STATE_FAILED    3       /**< The service failed to start */
 #define SERVICE_STATE_STOPPED   4       /**< The service has been stopped */
 
+// Set to 1 when services should stop
+extern volatile sig_atomic_t service_should_stop;
+
 /**
  * Find a service
  *