Bug Fixes:
- Fixed map-server crashed caused by Wall of Thorn when atatcked by Fire element (with wide range skill unit, bugreport9102). Also reverted some changes in c046668
- Added config conf/battle/skill.conf 'arrow_shower_knockback', as follow up 453b6d0. Direction of Arrow Shower knockback depends on Arrow Shower location

Signed-off-by: Cydh Ramdh <house.bad@gmail.com>