[Killstack] Escaping

[Lachu]

I think that some units, like mounted or wheeled(like tanks) should have a chance to escape instead of be killed. Unit will escape into random tile. To process escape, game should consider:
- Unit have more move points than attacker (imagine attacker could follow after unit and kill it)
- Unit will have only some percentage chance to escape - if they didn't escape, it would be killed
- Unit must have special flag - maybe called fast
- Attacker doesn't have kill fast flag
- Unit isn't in terrain disabling killstack
(And of course - other unit on the same tile must be killed)

Reason is simple. Some ruleset may create unit types with very low defense points, but strong on attacking. Also player can move unit to regain hitpoints in tile with another unit.

[Lachu]

This idea is quite realized by this patch:
http://gna.org/patch/?6104