Allow topping off partially-loaded firearms using speedloaders

[chaosvolt]

Is your feature request related to a problem? Please describe.

So in a nutshell, I'd been looking at the code for this but I'm completely stumped. The idea I had here is, basically it makes sense that revolver speedloaders can only be used on an empty firearm, as you'd have to eject the cartridges inside it anyway. But for speed tubes, stripper clips, and the like it'd generally be more reasonable to allow topping guns off from speedloaders by default, and exempting revolver speedloaders specifically via an additional flag.

Describe the solution you'd like

So accomplish this, we'd need to edit whatever code is involved that prevents a partially-loaded gun from being seen as valid to reload. Then we'd need to do the following:

1. Have this code instead check for a flag to be given to a flag used for exceptions to the rule, like revolver speedloaders.
2. Change the error-checking thing Scarf edited in fix: allow speedloader capacity to be smaller than gun's #3126 to just stop caring entirely because it would no longer matter if the magazine holds more than the gun, as it should just be dumping ammo in until the gun's happy or until the speedloader runs dry.
3. Sanity-check the actual reload code to make sure it only takes the ammo from the speedloader it needs to fill the gun up, i.e. it doesn't fill the gun above its clip size and doesn't cause any excess ammo in the speedloader to vanish into the ether.

Additional context

This would also be useful for the gatling gun idea @hirohito1 had, because then we could have it use a 40-round integral mag fed by 20-round loading blocks. That's how the most common style works IRL, being unable to fill up the gatling until it's completely empty would disrupt the whole "it has low capacity but is much easier to just keep it going for sustained fire" shtick that'd give it a valid niche to exist in.

Additionally we could use this to give all speed-chute-compatible shotguns access to all shotgun tubes, because then it wouldn't matter how at all how many rounds are in the tube, so reloading from one would just hoover up shells until either the gun is full or the tube is empty.