rename functions

add_weapon_skill to add_skill_slot
lose_weapon_skill to lose_skill_slot
refactoring
rename "weapon_slots" to "unused_skill_slots"
add comment
move part of code from lose_skill_slot to lose_skill
add forget_skills
rename losespells to forget_spells
Rework amnesia - mind flayers now destroy spells and skills
speedy skill advance increases use counter