Make units get out of the way of falling units

without:
- depending on height fallen from
- the idiocy of diving for cover under a different part of the falling
large unit

Opened a bug report at http://openxcom.org/bugs/openxcom/issues/279 containing some cases to try out.

Update:

- large units that cannot escape are killed/destroyed
- small units are knocked out for one turn
- infected units and zombies become knocked out chrysalids
- proximity grenades are now checked for