[HL2MP] Fix ammo crate exploit

[speedvoltage]

Issue:
If a player tosses a grenade or any explosive at an item_ammo_crate and then switch to a crowbar before the explosion happens, the crate will open up, giving ammo to the player who triggered it, thus creating an unfair advantage. Plus, only the crowbar can open these crates, which means the stunstick can’t do anything with them.

Fix:
Implemented a check to ensure that damage is only registered from a crowbar and have also made the stunstick capable of opening this type of crate.