Add tb-set-uselinkdelays to switch between normal delay nodes and
link delays setup on the nodes. Add tb-set-forcelinkdelays to force a linkdelay (when uselinkdelays is on of course) to be inserted, even if the link was unshaped. This allows it to be controlled later, say via the event system. This is nice, cause in the current system an unshaped link cannot be later shaped. Note that these are currently require FBSD-47.