- Clean up of Unit::SetSpeed (mostly cherry picks from the 6.x branch):
- the opcode sent depends on the unit. until now, MSG_MOVE_SET_* were sent for every units which isn't like retail behavior.
- Removed the unused method parameter "forced" from Unit::SetSpeed
- Renamed Unit::SetSpeed to SetSpeedRate
- Removed the unused method parameter "forced" from Unit::UpdateSpeed
- Added utility method Unit::SetSpeed which take the new flat value.