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>