support for partially-filled/empty guns which still have magazines

[SomeguyManperson]

About The Pull Request

magazines and guns can now control how much ammo they start with, allowing partially-filled magazines and guns that spawn with a magazine that is empty (previously a gun could either spawn full or with no magazine). This can either be set by a variable (percent_fill) for mapping or when being spawned (new /obj/item/gun(mapload, ammo_mod))

Also added support for empty gun/magazines in cargo, which can't be directly touched with either method, with set_empty variable in the cargo pack. This is currently unused but I've heard people want something of the sort.

Why It's Good For The Game

Changelog

:cl: tweak: guns and ammo holders can now be spawned partially filled, no player-facing changes /:cl: