Make transfer points availabe for certain modes of transportation only

[christianschaeffer]

This should allow transfers e.g. only for personnel on foot or only for vehicles.

Related to #375.

[anonym-HPI]

Same could also be (should be discussed) able to allow only a hubschrauber (helicopter) to land or start at a transfer point (as they are probably slower at landing and starting but much faster at travelling distances). If this transfer point is marked with an H or so, same for transfer points only allowing personnel via a symbol could be disccussed.