Deathmatch mode enhancements

[cybersteel8]

Since this isn't exactly deathmatch, even though it follows similar rules, it could be called survival in the menu. This won't change configuration name and will not use the "custom" gamemode. Unless a real deathmatch mode is created, this will take its place. Think of L4D's survival mode.

Also opens up an avenue to customize the experience for a more co-operative experience (but not the cooperative gamemode 😛)

[cybersteel8]

This cannot be named survival since that's now an official mode. Umm... "Endless"?

EDIT: Survival should be fine, as the new mode is now identified as Danger Zone as per #14

[cybersteel8]

Why not have some fun with this mode, like if you get a kill you get health back or something? It's currently a really hard mode, and the bot AI is kinda dumb, they gotta roam around more. See what you can do.

[cybersteel8]

mp_teammates_are_enemies
When set, your teammates act as enemies and all players are valid targets.

Add these to deathmatch config so that it becomes free-for-all now that Valve has improved bot brains in FFA mode!