clean up weapon spawner code

[nemesis-vampy]

i have also fixed a bug that caused some guns to be invisible to demon players.

additionally, two minor balance changes:

• the assault shotgun gave 16 ammo (8 from the spawner and 8 from the gun itself). i decided to reduce this to 8 for consistency with the other shotguns.
• the dropped shotgun originally gave 1-2 shells. i have increased it to 4, like in vanilla.