add a generator to pull rc-local.service in

add a generator to pull rc-local.service in

rc-local.service acts as an ordering barrier even if its condition is
false, because conditions are evaluated when the service is about to be
started.

To avoid the ordering barrier in a legacy-free system, add a generator
to pull rc-local.service into the transaction only if the script is
executable.

If/when we rewrite SysV compatibility into a generator, this one can become
a part of it.