multiple mirror code && setup HEAD

[Robert Collins]

Setup HEAD should not segfault anymore. I think I've squashed that bug.
(I can do local/source only and net installs with and without gdb to my
hearts content).

Setup now allows the selection of multiple mirror sites, and will
download from them failing over automatically when an error occurs, on a
per-package basis.

If anyone wants a snapshot, I'm happy to put one up.

There's a bit more code to come in (including clickable categories), but
I hope to feature freeze HEAD by this coming weekend, and have it
crash-free for a new update shortly after that.

So go nuts on it :].

Rob