Implement essential policy

[zeenix]

busd will have to implement a policy. However, I don't think we need to go as far as existing implementations. Most of the policies are not super useful. More specifically, we'll want service-level policy but not method-level. Admins can specify which names can be owned by which users and who can make calls to them. In the D-Bus configuration XML language, this would mean enable configurations like:

    <allow send_destination="org.gnome.DisplayManager"/>

and not supporting send_interface or send_member:

    <allow send_destination="org.gnome.DisplayManager"/>
      send_interface="org.gnome.DisplayManager.Manager"/>
      send_member="GetRemoteHostname"/>

We should not error out on encountering unsupported configuration nodes though, but rather just warn about them.

Prerequisites: #78.