power: Simplify tallying the power value of items

power: Simplify tallying the power value of items

Item power value is basically number of items times the item power
weight.  For some item types, we add up the item numbers, then apply
the weight.  For lcms and hcms, we apply the weight, then add up the
weighted numbers.

Adopt the latter method for all types: change addtopow() to tally the
power value for all types instead of just lcms and hcms, and drop
gen_power()'s item power value computation.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>