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.
`player::assign_activity` incorporates the current moves (removes the from the activity and sets `player::moves` to 0), removing them here (and not setting `p->moves` to 0) actually removes them twice. Removing the current move points here also has no effect when the activity is resumed (the moves are than taken from the backlog).