Game server: allowed for cospre item bonuses (supported at this time: stat, HP, MP, AC & AP [and class-specific] bonuses). Resolves #212.
Also cleaned up the class checks to make it easier to implement the class-specific behaviour, and removed all direct SetSlotItemValue() calls; a couple of cospre bonuses require SetSlotItemValue() is run immediately before SetUserAbility(), this guarantees it.