server/: Build a proper interface for handling tunnel classes.

  * Introduce functions for enumerating the available tunnel types,
    looking one up by name, and returning a default tunnel.  This
    eliminates the direct access previously used by the admin code.

  * Introduce a registration interface.  This isn't necessary for our
    purposes, but, in case it's not been obvious enough, the recent
    changes have been directed towards making the server code suitable
    as a library, and a user of this library might well need a custom
    tunnel class.