Baby steps towards auto landing: Added DO_LAND_START and RTL_AUTOLAND

Added logic to allow landing via command from GCS or by arriving at DO_LAND_START mission item.

Also added an RTL_AUTOLAND parameter that will cause the plane to automatically begin to auto land after arriving at an RTL location (rally point or home point, depending on configuration).

See this thread for context:

https://groups.google.com/forum/#!topic/drones-discuss/XmVM9v4kIeo