AI tell their greatings even when player is moving (like morrowind).
Greating reset distance was greatly inflated comparing with morrowind.
Implement AI greeting states.

Greeting consist of 3 phases:
- none - default one, greeting state can only change to "in progress"
  when near enough and some time passes
- in progress - NPC says his greating and rotates toward player,
  state can only change to "done" after some time
- done - rotation is stoped, after idling can go away from player,
  state can only change to "none" when player and NPC are faraway
Add grace periods for player greeting.

Add delay for NPC greating. NPC ignores greeted player after some time.
Fixes bug 1503.