Make m&f brand gen more sane

Math notes:

The chance of the brand not being set (that is, still being SPWPN_
NORMAL) by the time the SPWPN_VORPAL if() was as follows:
most weapons: 24/25 * 24/25 * 24/25 	= 88/100
morningstar: 3/4 * 3/4 * 19/20 * 88/100	= 47/100
eveningstar: 1/2 * 47/100		= 24/100

The odds of that brand being overridden is 55/100.

This code is really goofy