BotManager: Add option to disable spectating while bots are in the game to fix randomly adding or removing bots when players switch spectator state

Also fix warnings introduced in recent change when the game is not a valid deathmatch
Add shield gun gametype dummy to allow having all shieldgun deathmatch maps under a different category in map vote

Based on Elimination2k4 gametype

You need to rename the "DM-SG-..." maps to "SGDM-..."