Our firewall rules rely on the ability to resolve names.  Thus, the
manipulation we do on the firewall node to move the control net from a
NIC to a bridge with the NIC inside it must save all the routes, too.

If this code used to do that properly for Ubuntu 10, it certainly does
not for Ubuntu 16, for one of potentially multiple reasons.  So I wrote
some fun code to do its very best to save all routes related to the
control net device (including the default route) in such a way as to 1)
obtain *all* routes before tearing *any* down any; and 2) to ensure that
the transformed routes can be safely restored.  (This will then work for
any testbed with an obtuse control net setup in which the default route
is insufficient to reach boss, not that that will ever happen.)  This
code does its thing on both the enable/disable paths, of course.  Here's
the relevant details from the code:

    This is very, very messy.  We have to save the
    existing routes for $pdev, but in an order that they
    can be restored without failing, before we move
    $pdev into br0.  Then the restored routes must be
    rewritten in terms of br0 instead of $pdev.

    Finally, we have to collect these routes prior to 1)
    assigning $pdev's IP to br0, and 2) moving $pdev
    into br0, and 3) prior to *deleting* any routes.
    All these conditions seem necessary to me.

    ip route show does not necessarily display routes in
    restoreable order.  In particular, the thing that
    bites is that the default route is (now, as of
    Ubuntu16, at least) displayed *prior to* the scope
    link (broadcast route) for a device).  So, we first
    harvest the scope link routes for $pdev (i.e., the
    natural ones that come into being as a side affect
    of assiging an IP address to a link); then we
    harvest all other routes associated with $pdev; then
    we remove all routes associated with $pdev.  Then we
    do the mucking about with bridge membership and
    transfer IP from $pdev to bridge, and finally,
    restore the routes we saved (modified to br0, of
    course).