Plague 0.5 update: builder-to-server communication

[Dan Williams]

Hi,

I checked in the last bits for builder-initiated communication earlier
this week to cvs HEAD.  This means that builder instances no longer have
to run _any_ server (XML-RPC, HTTP, or otherwise), and that builders can
be behind a firewall without problems.  I call these "Active" builders,
as they initiate all communication to to the build server.

It appears to work with light use, but there are still bugs and
edge-cases to work out.

Next steps include beating down bugs introduced by this refactor,
finishing up the server-to-builder ("Passive") communication code,
writing documentation, and getting more testing.  I'd also like to make
sure the plague directory structure is FHS-clean

Somebody is sending me an SVN patch too.

Dan