[airstrike] BUG: engine/math.c:vnormalize()is not vector normalization

[Ulf Ekstrom]

> Hi,
> 
> while working on the seeker AI I found out that vnormalize() from
> engine/maths.c is not vector normalization.

Hi! It's really strange that we didn't discover this bug earlier;
thanks for the find! It should be sqrt(t) of course. Maybe now I can
make the physics better as well. Please commit the fix, I will also
test a bit for things that break.

Ulf