-Codechange: Remove _avail_aircraft
Its name is misleading. It is rather _avail_airports, but then only some of them, which leads to inconsistencies when using it.
Further it is unnecessary to store it in savegams.
-Codechange: Remove the unnecessary attributes Station::{bus,lorry}_tile_obsolete by replacing them with a scan of the map for existing road stops when loading old savegames
Change the signature of Swap() to be less error prone, i.e. pass the variables to be swapped by reference instead of passing pointers to the variables.
Just do Swap(x, y) instead of Swap(&x, &y). This prevents accidents when the variables are pointers.
Only doc landscape_grid.html has been updated, landscape.html will be done latter.
-CodeChange: Replace a direct access to m6 with SetTropicZone in InitializeLandscape, as it is the only part of m6 still untouched without the brute-forced m6=0
