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 simple paths, the intent being
  that we will probably distribute images offline (say, at night) so
  that when a node reboots it doesn't actually have to wait 60 minutes
  for an image to download. I have not added any server side support
  for this yet though. Maybe later this week.

* Other bits and pieces and fixes to make this work.

"select" to a "select distinct" on osidtoimageid since there will be
multiple osid entries for combo images.

working on this a few weeks ago when I got sidetracked by the pubsub
project.

  register_globals=1 to turn POST/GET/COOKIES arguments in local variables.
  This is known to be a terrible security risk, and we keep saying we are
  going to fix it, and now I am. In order to accomplish this on a
  transitional basis (since I don't want the entire web interface to stop
  working while I debug it), and because the code just needs the cleanup, I
  am doing it like this: Each page will sport new declarations at the top:

	RequiredPageArguments("experiment", PAGEARG_EXPERIMENT,
                              "template",   PAGEARG_TEMPLATE,
                              "instance",   PAGEARG_INSTANCE,
                              "metadata",   PAGEARG_METADATA,
                              "osinfo",     PAGEARG_OSINFO,
                              "image",      PAGEARG_IMAGE,
                              "project",    PAGEARG_PROJECT,
                              "group",      PAGEARG_GROUP,
                       ...