Provide support for a default site if no header is provided

[ieugen]

It is my understanding that pcp-server implements a form of virtual webhosting where the site you want is supplied via a header.
Apps should send a header with the document root.
I believe a nice feature would be to provide a default website option.
When enabled and configured, it should allow for requests without a default website and use the one configured.

Side note: In http, the normal header for this case is Host: but not sure if this can be used here. It might be the case that this header is also configurable by the user ?!