Copter: when leaving AUTO only stop mission if running

This resolves an issue in which the mission would not automatically
start from the beginning if it had previously been run to completion
Copter: reset mission when disarming
Copter: minor comment update
AP_Relay: add -1:Disabled to list of param values
Rover: remove unused RELAY_PIN definition

This is now part of the AP_Relay library
Copter: add Pixhawk pin to RSSI_PIN's @Value

Also prepend APM2 to existing definitions
Rover: add Pixhawk pin to RSSI_PIN's @Value

Also prepended APM2 to existing values.  Added A2 to list to be
consistent with plane and copter
Plane: prepended APM to RSSI_PIN's @Values

This makes it consistent with rover and copter and clarifies where the
pin is on the Pixhawk