Disambiguate "gunner"

BUR #6521 has been approved by @evazion.

create alias gunner -> gunner_(sekaiju)

For some reason we've been using this rather generic name for a very specific game's class. I'd do a rename, but for some reason there's a post limit for that.

The bulk update request #6521 (forum #193043) has been approved by @evazion.

BUR #6541 has been approved by @evazion.

remove alias gunner -> gunner_(sekaiju)

The other shoe drops on the rename.

The bulk update request #6541 (forum #193133) has been approved by @evazion.