Some minor pkill-related changes

News #1126 posted by Gonnil on 05/15/19.

These changes were a result of PKC vote. All changes affect both peaceful and
deadly characters, but these changes are all geared toward pkill on either
type of character.

- Thieves are now able to use the backlash skill.
- The uppercut skill minimum position has been changed from standard to
- All arena rooms have been changed to be outdoor Forest sector type. (If you
notice an arena that has not been changed, let an Immortal know.)