AGM_Hearing: Refactoring of the volume processing routine and support for earplugs

Refactored the volume processing routine, spawning a single script thread responsible of updating the volume. This solves the problem of competing threads lowering and rising the volume alternatively, which created volume spikes during full-auto fire with some rifles. It should also marginally increase performance.

Deafening effect is increased, and a cuadratic relation between deafness and volume is introduced.  Coeficients are such that, without earplugs, rocket launchers produce total loss of hearing for a few seconds.

Most hand weapons produce a reduccion of aprox. 0.25 to 0.75 volume reduction when in full auto. Ear ringing is left unaltered.

Support for X39 medsys earplugs is added, without affecting anything  when x39 is not present. With earplugs the effect of deafness and ringing is greatly reduced, by decreasing the noise strength.