Aerospace landing fixes

[neoancient]

Fixes several issues related to aerospace units making landings:

1. Units making vertical landings only apply half the terrain modifier (TW, p. 87).
2. I added terrain modifiers for advanced terrain from TacOps.
3. When the landing path includes rough or rubble terrain, the landing unit takes damage (TW, p. 86, table).
4. A unit that lands in water of sufficient depth (2 for dropships, 1 for other units) is destroyed (TW p. 87, which refers to p. 82).
5. A dropship that lands in depth 1 water is immobile for the rest of the game (ibid).

I exempted LAMs and units with a flotation hull from water destruction.
Fixes #4743

[SJuliez]

There's a typo in "landing gear" of the messages.properties. The footnote text in the table of TW p.86 suggests adding only the highest terrain modifier which would contradict the text of p.87. Seems to me to make sense doing it like the text says. So, all ok.