added friendly fire and self damage handling to dmg2score

added friendly fire and self damage handling to dmg2score
some players might try to cheese or avoid giving other players score and
thus suicide or do self damage, they are now punished for it but they
are not punished while having a logged attacker so getting pushed into a
void doesn't punish the dying player as long as pusher is given score. I
assume DEATH_FIRE is an environmental hazard but I don't know any maps
which'd have it for testing though if it's a weapon effect which
registers an attacker it's irrelevant except for being unused code.