widearea reloading support.

* New slot in the images table to store an access key which remote
  sites must provide in order to download an image (via https).

* tmcd returns a different kind of ADDRESS field from doloadinfo.
  Instead of the multicast stuff, return a URL that points to boss'
  web server. The URL is of the form:

   https://www.myemulab.net/spewimage.php?imageid=10013&access_key=abcdef

  which as you can see is fully specified; the client does not need
  to know anything else.

* New webpage and backend scripts appropriately called "spewimage"
  which also includes support for the http HEAD request (from wget) to
  avoid downloading images that are already on the node. I just
  learned about this HEAD request stuff today ... but otherwise these
  operate as expected, spewing the image if the access key is provided.

* Changes to rc.frisbee to deal with remote loading. In addition to
  URL support, I also added support for...