Fix - Do not allow players to attack while sitting

While working on roBrowser I figured out there is no check server-side to block a players to do a *regular* attack while sitting.

![sit-bug](https://cloud.githubusercontent.com/assets/4102403/2707667/f980e84a-c4a3-11e3-87b2-5fa610b6d0b2.jpg)

The attack do not reset the "sit" stat, so you can't walk or do skill but you'll keep your awesome double hp/sp regeneration.
It can be abused (specially by long range players) to regenerate while attacking monsters.