I'd like to enhance Roundup's separation between the user interface and
the tracker instance behavior. In particular, while detectors are fired
from within the hyperdb, actions right now are only available to the CGI
interface.

I'd like to suggest a series of enhancements that allows users to
provide actions that are callable via different interfaces (such as XMLRPC).

As a first step, here is a patch that allows cgi.client to multiplex
requests to an XMLRPC request handler, if the content-type of request is
text/xml.

A couple of comments on this patch:

1) I had to slightly refactor bits to break out of a dependency loop. (I
moved the 'UsageError' definition from admin to exceptions, so the
xmlrpc module could use it without becoming dependent on cgi itself.)

2) I now realize how badly I designed xmlrpc.RoundupServer. In
particular, as it does its own authentication, I couldn't use it from
within cgi.client. In this patch I added a new 'RoundupServer2' that
corrects that. I wonder how great the chances are that the original
RoundupServer is used directly by anybody, and thus, whether we could
break backwards-compatibility by just replacing it by the new one.)

While I have working prototype code of a new roundup.actions module,
callable both directly via XMLRPC, as well as bia a bridge in
roundup.cgi.actions, I'd rather keep that as a separate patch.

How does this look ?