defineskill and undefineskill changes
Description (got lost in squashing):
Duke3D: Allow undefining a skill without undefining everything above that level.
For example, it is now possible to undefine all skills except "Come Get Some".
Furthermore, you can now undefine all skills, in which case the menu is skipped, and the default skill level is used. This was always the intended behavior, but crashed the game due to a bug.
The default skill is defined by: https://wiki.eduke32.com/wiki/Default_skill