GCS_MAVLink: merged upstream changes
GCS_MAVLink: added MAV_CMD_DO_LAND_START

see discussion on drones-discuss
GCS_MAVLink: re-generated headers
AP_Mission: Added support for MAV_CMD_DO_LAND_START
Plane: Added support for MAV_CMD_DO_LAND_START
Plane: Added parameter RTL_AUTOLAND and implementation.
AP_AHRS: make get_position() const

This allows use from within AP_Mission
AP_Mission: avoid the AP_AHRS cast

get_position() is now const
Plane: improved check for reaching RTL point for auto-land
Plane: switch to AUTO on a DO_LAND_START MAVLink command
Plane: improved RTL_AUTOLAND docs
AP_Mission: fixed logic in jump_to_landing_sequence()

see comments on drones-discuss
Plane: ignore DO_LAND_START in missions
Plane: prevent calling jump_to_landing_sequence() too often