Protect teammates from splash damage when friendly fire is on

[roncli]

Rebirth has implemented protecting teammates from splash damage when friendly fire is on, we should as well since that is an annoying bug that makes friendly fire almost useless in certain games.

The problem with Rebirth's implementation, however, is that it prevents suicides by missile, and disallows deaths by teammate's pyro explosion. We need to discuss whether we want to implement this exactly like Rebirth does, or roll our own that makes more sense for the multiplayer community.