SotBE S6: modifications to map (water hexes only)

Ensure that the transports can get to within one move of their landing
sites by moving on deep water surrounded by deep water only.
SotBE S6: move transport ship movement code to Lua AI

Only the moves are done by the AI at this time, as it is currently not
possible to determine the best hexes onto which units should be
disembarking in a replay-safe way from within the AI.  We are working
on that functionality though, so no WML code doing this is added at
this time either.