exclusive flags

[mayth]

We can set multiple flags for the one challenge. All flags are valid for that challenge. "Exclusive flag" will change this behavior. If a player captures the exclusive flag, any other flags which belong to the same group will be disabled. For example:

1. 4 flags are available: Group A { flag1, flag2 }, Group B {flag3, flag4}
2. A player captures flag1
3. He/She cannot capture flag2. flag3 or flag4 can be captured.