When an exit node has been set and a new default route is added,
create a new rtable in the default rdomain and add the current
default route via its physical interface. When control() is
requesting a connection not go through the exit-node default route,
we can use the SO_RTABLE socket option to force it through the new
rtable we created.
Updates #17321
Signed-off-by: joshua stein <jcs@jcs.org>