update: Make upd_buildeff() return cost, drop parameter

update: Make upd_buildeff() return cost, drop parameter

Since changing *sp is safe now, we can move the update of
sp->sct_effic into upd_buildeff().  This frees the return value; use
it to return cost, and drop the parameter.

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