The hero isn't the only adventurer seeking the Amulet. It's clear
from various other events in the game that others have been there
beforehand. As such, we can expect many of the traps on the first
few levels to already have been triggered repeatedly by questing
adventurers.
This commit allows for the creation of adventurer corpses in
early-game traps, together with a small amount of cursed junk
(i.e. a miniature bones pile) and any items created by the trap
itself. On dungeon level 1, this is guaranteed for the vast
majority of harmful traps, in order to avoid near-unavoidable
deaths in the very early game due to not having enough max HP to
survive a trap hit.
Wizard mode testing shows that this case doesn't trigger very
often; maybe once a game on average. (Traps are rare on filler
levels, at least early on, and many types of trap would leave no
evidence, e.g. a teleportation trap won't kill people on its own
square.) As a result, the balance impact from the actual items
here is likely to be minimal (it may help out ranged combat roles
slightly but they could do with the boost). The main change,
therefore, is to reduce the number of unfair very early deaths
(replacing them with fairer "you shouldn't have investigated
what created that corpse!" deaths).
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