Add ds_dropammo CVar to control zombie ammo drops

* Added MaybeClipDrop, MaybeBigClipDrop and MaybeShellDrop
* Checks ds_dropammo and spawns the correct amount of ammo if true
* Improves gun balance when using level mods that eschew ammo for zombies

This branch and the doomhealth branch don't play nice with git merge, there's something about the constant definition files that gnu patch doesn't seem to like. So if you decide to take them both, I'll have to rebase and fix merge errors before the latter one will apply cleanly.