ArduCopter: Guided mode packet has option to not set_mode(GUIDED)

This change is required for implementation of a better followme device:

Allows a GCS to send a packet to update the guided mode waypoint without causing the autopilot to enter guided mode. This means the GCS can update the guided mode waypoint as often as it wants without worrying about taking mode control away from the pilot's RC transmitter, or a separate GCS.

Haven't flight tested this change, but its simple enough.