GCS: Add Rangefinder message from sonar altitude

I've had this patch sitting around for a few months, figuring that it would become obsolete if/when all the rangefinder stuff is tidied up. There has been at least once instance on the mailing lists recently though where a user has been confounded that ticking the 'sonar alt' box in Mission Planner doesn't actually give a reading, so I figure this patch has value in the short term anyway.

At the moment, the copter sonar set up does not have access to the raw sonar readings so this code sets the voltage field to zero.