fix: don't spawn grenade launchers installed on magazines

[chaosvolt]

Purpose of change

So recently I spotted a weird lil error: when a gun spawn (like zombie soldier drops) generates the grenadier variant, it also spawns M203s attached to the spare magazines that spawn, leading to this oddity when you try to unload them:

Along the way I spotted another smol mistake that I figured is minor enough to bundle in this.

Describe the solution

1. Redefined contents of military_standard_grenadier to define grenadier-specific versions of the sub-itemgroups in military_standard_assault_rifles instead of using the itemgroup wholesale, so it won't also apply the gunmod to the spare mags derp.
2. Defined the above-mentioned variants of the relevant everyday-carry itemgroups, which spawn the relevant gun with military_accessories_grenadier added to them, then the relevant spare mags.
3. Misc: Fixed clothing_tactical_leg having two calls to spawn hand grenade pouches instead of one call to that and a call to grenade ammo pouches.

Describe alternatives you've considered

Screaming.

Testing

1. Checked affected files for syntax and lint errors.
2. Ported JSON changes to test build, spawned in and debug-killed a bunch of zombie soldiers.
3. Sifted through for grenadier-variant weapon drops, confirmed the rifles correctly have a grenade launcher attatched while the spare STANAG mags don't let me unload M203s off them.

Additional context

Checklist