AC_PosControl: Protect from divide-by-zero in get_stopping_point_xy
AntennaTracker: fix documentation
Copter: remove DRIFT and SPORT from manual_flight_mode function
Copter: add land_complete to fence disarm check
Copter: use ap.throttle_zero instead of rc_3.control_in in auto_disarm_check
Copter: auto-disarm if land complete regardless of mode
Copter: add mode_allows_arming function
Copter: move all arm check logic into arm_checks
Copter: allow arming in GUIDED only from GCS

Also changed mode_allows_arming function to accept arming_from_gcs param
Also remove AUTOTUNE from arming list
Tracker: only run servo updates when safety is off

Largely removes the sudden jerk that occurs when the user pushes the
safety switch
Tracker: update version to 0.2
Copter: Start logging on arming attempt, rather than on successful arm
Update CONTRIBUTING.md
Update CONTRIBUTING.md
Plane: start preparing for 3.1.2
autotest: update compiler
AP_RangeFinder: auto-update PX4 ll40ls max/min distance

this allows the range of the Lidar to be set by the user using
RNGFND_MAX_CM and RNGFND_MIN_CM
Copter: enable Guided velocity controller for Pixhawk
Copter: remove unused pilot_yaw_override flag
Mission: support GUIDED_ENABLE and GUIDED_LIMITS

This replaces the ardupilot only NAV_GUIDED command.
Also remove support for NAV_VELOCITY mission command which will be
replaced by SET_POSITION_TARGET non-mission command.