Port facehugger code from Colonial Marines
Created by: gbasood
Fixes #7073 (closed) in theory Should make facehuggers actually smart about target picking
- Facehuggers will not tear eachother off of eachother's hosts.
- Facehuggers now have a health var. They die when health <= 0. Not sure about implications of that.
- Various values for facehuggers have changed. Not all of them match Colonial Marines, some were taken from our code.
- Facehuggers will no longer impregnate a person who has already been impregnated.
- Facehuggers instantly attach to anyone who is adjacent to them, and will also path to nearby people to some degree. (range of 4, select targets on process() call.)