get_primary_port
Description:
Gets the name of the primary port controlling this Modem.
Warning: 
      
        
    
    The returned value is only valid until the property changes so it is only safe to use this function on the thread where this was constructed. Use dup_primary_port if on another thread.
Parameters:
| this | A Modem. | 
Returns:
| The name of the primary port. Do not free the returned value, it belongs to this. |