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
Progress loop if unit could not escape

because looping indefinitely is not going to help anybody.
Knock out any units that cannot escape falling units

Also ensure crush victims to not hide their crusher.
Large crush victims are killed.
Tanks do not die of fatal wounds

They stoically accept all suffering.
Correct check for unit displacement

when resetting tiles after a unit becomes a corpse
Fixes falling unit sometimes being misidenitfied

.. as a fallen onto unit, handles multiple simultaneous falling units,
and replaces 1 x embarrassing typo.
Check if falling units trigger proximity grenades
Handle falling onto zombies

Convert zombies to their spawn unit if they cannot get out of the way of
a falling unit.
Merge upstream fixes for proximity grenades
Really merge upstream fixes for proximity grenades

who knew merging could be so much fun?
Set proximity grenades to be active on turn set
Merge pull request #655 from cfailde/falling

Make units get out of the way of falling units