Allow for tuning of Pack-a-Punch ammo prices

[tanuki-billie]

Description

Currently, the cost of buying ammo for a PaP'ed weapon is hardcoded at 4500 points. This proposal requests a way to change that via a KVP field in the buy_weapon entity, similar to how weapon and non-PaP ammo prices may be tweaked. It should still default to 4500, but mappers may want to change this (i.e. being able to increase the price of universal ammo purchases to offset the ability to replenish wonder weapons at will).

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[MotoLegacy]

nzp-team/quakec@de012d4