More flexibility for WasAvoidable

Allows custom weapons to easily set whether they were avoidable.
Removes verbose hardcoded special case for C4 traitor vs traitor damage karma
Updated C4 to use new avoidable damage system

Updated C4 to use more flexible avoidable damage system
Merge pull request #663 from YMIndustries/master

Make system of "avoidable damage" received by a Traitor accessible to custom weapons