Fix drill charge usage

The drills weren't taking the variable usage cost into account (either
the per-type base cost or the per-mode multiplier) when deciding whether
they have sufficient charge to use.  This could cause them to overshoot in
charge usage, although they would then clamp to zero rather than record
negative charge.  Also, for the Mk1 drill where the cost was assessed
correctly, the drill would refuse to discharge to exactly zero charge.