New DMFlags "sv_noallyicons" and "sv_noenemyicons"

This adds the DMFlag sv_noallyorenemyicons, which forbids any ally or enemy icons from appearing over a player's head. As mentioned in https://zandronum.com/tracker/view.php?id=4343, the enemy icon can give players unfair advantages in some situations. Furthermore, now that ally icons can appear in cooperative game modes, it may negatively affect the gameplay of PvP survival mods like Who Dun It and Zombie Horde.