A couple of fixes for reaction fire on falling units

The current situation is either
- shoot out floor over an alien's head, reaction fire (!), then gravity (!!)

Akseli is about to go all commando on a snakeman 
![screen010](https://f.cloud.github.com/assets/1824834/1432161/092e4498-40da-11e3-8ecd-e1bedf198c7e.png)
.. he was unprepared for the snakeman's reactions:
![screen011](https://f.cloud.github.com/assets/1824834/1432173/47829d3e-40da-11e3-9210-d98d40b9f15f.png)
Ouch!  That left a mark.

Or it's 
- shoot out floor, gravity, land near an alien, no reaction fire (!)

Knowledgeable Akseli
![screen014](https://f.cloud.github.com/assets/1824834/1432296/fc6557fe-40db-11e3-9f73-e9833fe5add0.png)
.. lands a couple of squares away in fall, I mean full, view of a Snakeman
![screen015](https://f.cloud.github.com/assets/1824834/1432302/07e88830-40dc-11e3-8507-124dca43a928.png)
Spin, shoot (pew, pew, pew), kill, celebrate.

Let's change all of that by doing reaction fire after a unit has landed and definitely not before gravity.