Martial arts buffs: scale the die size with skill

[copperwater]

YANI #1512 (modified). Don't use a base d4 for martial arts attacks. Increase the die size by 2 for every level beyond Basic: d6 for Skilled, d8 for Expert, d10 for Master, d12 for Grand Master.

Unlike in YANI #1512, leave the flat skill bonus damage intact, and don't touch the die size for bare hands skill.

[copperwater]

This needs to be revisited in light of vanilla's recent change giving martial arts extra attacks (which I intend to incorporate), which has boosted the overall expected damage: https://nethackwiki.com/wiki/Martial_arts#To-hit_and_damage