items: apply 0.5s spawnshield to all loot drops by default

items: apply 0.5s spawnshield to all loot drops by default

This prevents immediate pickup of the dropped item.

Previously powerups used this mechanism, whereas thrown weapons could be
picked up immediately by everyone except the person who threw them.

It will be consistent for most/all loot items now, and perhaps nicer because
players should see the items in the world before picking them up so it
might feel more deliberate and less random.