Finish the switchover to Andrews & Arnold.

  * Remove the old HE netblock.  I've switched the house over to using
    the A&A IPv6 netblock throughout because multihoming just isn't
    going to work well.

  * Remove the `aaisp' network name now that I've decided we're not
    doing parallel running.

  * Allocate a little gateway network for the PPP-terminating router.
    It turns out that if I don't do this then it uses a completely bogus
    default source address for the PPP interface.

  * Incidentally, fix the NTP-server netblocks to include the Jump range
    as well as the house range.