Add HTML mode for RPC calls

[Kris Deugau]

As an alternative to embedding the core codebase in something else, and having to keep the database connection info up to date and the database accessible, allow a remote system to request pages or page fragments from the RPC listener.

It may be better to just create a separate listener to handle the HTML bits.

[Kris Deugau]

Tentatively tagging for 1.6, since this would help push ahead on getting the more abstract "integrate DNS management with foo" process