Martial arts buffs: second attack at Grand Master

[copperwater]

Evilhack commit fce780e implements this, but needs some work to port it to xNetHack. One sticky bit is that the second hand should never apply the sear damage from the material of a ring worn on the hand of the first attack. The code is currently set up to hit the monster with the hand that delivers the most favorable sear damage on every attack, so adding another attack with the other hand will complicate this.

The second attack should be canceled if the first attack in any way results in your death (punching a cockatrice, passive attack, etc).

One possible modification is rather than grant this exclusively at Grand Master, make it possible though unlikely at Expert, moderately likely at Master, and fully in effect at Grand Master.