As per discussions with David Zeuthen, alter the build system so that we
can have different authentication frameworks for the authorisation
databases.

For now, the dummy database will only accept 'none' for the authentication
framework (this will be autoselected if not specified, and configure will
throw an error if any other framework than 'none' is specified is passed
in).

For the default database, the only available framework for now is 'pam'
(as with 'none' and dummy, 'pam' will be autoselected if specified as the
framework. If 'none' is passed as a framework, configure will reject this
and fail).

PAM specific code is now also marked with POLKIT_AUTHFW_PAM, so that it
can be easily compiled out if other frameworks are added in future.