[python] add pprz_env with usable default IVY_BUS

- pprz_env.IVY_BUS is either the env variable IVY_BUS or appropriate default (checks for OSX)
- some fixes for udp_link

should fix #1495 and the python part of #204

sw/lib/python/serial_msg_interface.py

@@ -39,7 +39,7 @@ class SerialMessagesInterface(threading.Thread):
     def shutdown(self):
         self.stop()
 
-    def __init__del__(self):
+    def __del__(self):
         try:
             self.ser.close()
         except: