PATCH: Allow tracd to serve projects from the URL root

[pacopablo]

With external webservers, it is possible to configure Trac such that it is served from the root URL. However, tracd will always display a project listing, even if only one project is beign served.

This request has come up on the mailing list previously. The response from Matt Good was that another option should be provided to tracd.

Attached is a patch that provides the --serve-root option. If specified, then it will not show a project list and simply serve the project listed.

I'll admit that my approach might not be the best. However, I couldn't think of a better way to get the necessary info to the required objects. So any feedback on the patch is very welcome.

[pacopablo]

duplicate, see #3506

Sorry about that. I don't know why it got submitted twice.