This gives a clean break between the transfer request and the type of
transport allocated to make way for transports that need to switch
types (to support driving to a port, then getting on a ship, to a train,
then back on the road, etc).
https://github.com/Khopa/dcs_liberation/issues/823
The previous flight plan only makes sense if the convoy will make it a
significant distance from its starting point. At road speeds over the
typical mission duration this is not true, so we can actually plan this
as if it was a strike mission near the origin point and that's close
enough.
There's some cleanup work to do here that I've added todos for.
Fixes https://github.com/Khopa/dcs_liberation/issues/996
