Fix #59 weapon auto-equipping

[szapp]

These changes should fix the bug #59. Quoting #59 for a minor issue:

A few words about the fix. It will prevent NPCs from equipping a weapon when they already have one of the type equipped. That means in particular:

1. If an NPC is created with a melee weapon, they will stick to this weapon for the entire game, unless explicitly exchanged through the scripts.
2. If an NPC is created without a ranged weapon, they will equip a ranged weapon should they ever acquire one (e.g. through trading with the player).

In short: An NPC willl auto-equip a ranged and a melee weapon if available - but only the first time.

So, you could argue the fix only works to a certain extend. In most cases, the first sold ranged weapon will be "lost" for most traders.

Originally posted by @szapp in #59 (comment)