Migrate speed, damage/armor and firepower modifiers to integer

Current state:
SpeedModifier seems to work fine.

DamageModifier currently does about twice as much damage as it should, probably a misplaced division op, haven't found the error yet.

FirepowerModifier leads to zero damage of tanks vs. infantry for example, so there's probably a misplaced division op as well.
I had some trouble coming up with an integer formula that gets the same effective result as the old float formula. Since @pchote is redoing Upgrades in #6110 anyway, I was hoping a 'cheap' stop-gap solution would be acceptable this once.

If not, I'll need help with that formula, has been ages since I've needed this kind of math knowledge.