Add PlayerModifiers.CanDisarm

[NearHuscarl]

One of my bosses that I am writing uses a grenade launcher, and because the weapon require reloading after every shot and is mostly used when aiming, it's very easy to disarm the boss. But I want the player only obtain the weapon after killing it as a reward. Currently my workaround is to re-equip the weapon and destroy the dropped one, but it has an annoying equip weapon text. So it'd be nice to have a flag to indicate that this bot can never be disarmed.