Make polearm brand generation sane (?)

Chance of SPWPN_NORMAL by the time SPWPN_VORPAl was reached was
almost exactly 4/5; (1/5 * 4/5) + (1/5 * 1/5 * 1/6) is roughly
1/6 = 16.6...%, which * 69% (the chance of that brand not being
overriden by a later brand) is roughly 12%.

hooray