Zlave tweaks

Tried to improve on the messages a little bit, and make the morale check a little more gradual instead of it being all-or-nothing.
You should probably store the index of the corpse item on the map, as it is done in `game::butcher`.

For non-resumable activities:
change `player_activity::is_suspendable` to return false for your activity. This is used in `player::cancel_activity`, false indicates that the current activity is *not* stored in the backlog and is therefor not resumable.