Allow Syringe Zombie to have random drug effect

[UndeadZeratul]

Upon spawning in, a flag could be set to randomize what drug effect is used when getting booped. The sprite could even be translated based on that flag, since the serum in the syringe is green.

Stims could inadvertently cause the player to overdose and have a heart attack.
Zerk could enrage the player and/or cause a heart attack.
Poisonous attack could behave the way it does currently by stunning the player.