Code review comment for lp://staging/~vbocek/ubuntu-system-image/support_http

Thanks very much for this branch. I'm going to merge it to trunk after some
tweaks, and fleshing out of the test suite.

I ended up making these changes to your branch:

  * Renamed DISABLED_PROTOCOL to DISABLED
  * Some minor code style issues.
  * Allow for the [system] section to be missing from a channel.ini file.
  * Bag class API change: Setting an attribute via the setitem API no longer
    makes that item read-only. IOW, only keywords set in the constructor are
    read-only.
  * Flesh out tests.
  * Tweak the manpage.

Once I've merged this to trunk, you can take a look at the specific diff
there. Please let me know if I've missed anything or if you have any
questions or comments.