VtolPathFollower: support landing in paths

This makes do_path method is now aware of the particular mode for
the current waypoint and can call other methods such as landing.
PathPlanner: support Waypoints using LAND mode
VtolFollower: landing should stay INPROGRESS

Since landing only makes sense as the last element in
a flight plan there is no sense in setting it complete
hitting the landing location.
VtolFollower: add ability to point heading along path

This looks forward in the direction of the path, provided
the desired velocity is greater than 1 m/s. This should
prevent too much spinning around while hovering. However,
the MaximumRate for Yaw needs to be set to something
reasonable where hitting full stick isn't a problem.
VtolPathFollower: if cannot control this frame set critical error

Having this a warning is unsafe as this condition will result in
no follower. They should definitely not be taking off like this.
Firmware: update copyright headers
RTH: whitespace fixs and other things from review