Add natives to properly use TFCond_TmpDamageBonus

Relevant changes to TF2_AddCondition included:
"Inflictor" changed to "Provider".
Provider is now allowed to be any entity (for instance, dispensers).
Moved actual call to TF2_AddCond to its own function, called by
TF2_AddCondition native (as well as by the TmpDamageBonus natives).
The 12 should be a named constant, rather than a magic number.