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.
About resume activity.

I think this activity should be done without the opportunity to continue (but abortable as now), because we can select other corpse to make zlave, e.g. more complexity. That is beginning to make a zlave from usual Zombie (hign chance of success), then cancel, and begin to make a slave from the Hulk. Exploit. Or no?

May be before `assign_activity` we shall call `cancel_activity` or reset paramaters (variables) of old activity.