Fix Radius Enchantments hitting spectators in Arena.

Currently players can cast a RE (i.e. sandstorm) in the Arena, end the match and leave and the enchantment will hit the spectators even if a fight is not in session. I've put some Arena checks in radiusench.kod and REs now behave appropriately in the Arena.

Spectators can see the battle messages but will not receive icons (or be hit by effects).