dbus: extract 'dbus-common' sub-package
    
    Create a new sub-package called 'dbus-common', extracting the XML
    configuration and setup files from 'dbus-daemon'. These files might be
    used by alternative D-Bus Messages Bus implementations, hence we want
    them as a separate package.
    
    This also pulls in the 'dbus' sysuser.d setup file, since this uid is
    referenced from the xml-configuration of the system bus. Hence, it is
    part of the configuration and required to be used by any compatible
    implementation using the official XML configurations.
    
    Signed-off-by: David Herrmann <dh.herrmann@gmail.com>