fix burst fire weapons having their wear go over max_wear and breaking

[Elkien3]

I copied the wear code from fire() and edited slightly to be multiplied by burst.

[ClobberXD]

All *_fire methods end up calling fire in their own way (or at least that's how I intended them to work), but calculating wear in fire doesn't work for burst_fire, as it can't return the wear-adjusted stack from within minetest.after.

Even if this works, this results in code duplication. Would it be a better idea to move the wear-calculation code up the call stack (maybe in on_lclick?), to ensure that it gets called for all modes incl. burst?

[Elkien3]

Good idea, I'll see about that.