🍒improve dbus session-local hack

• do not include system services, they have no business being on the session bus
• ensure the standard services are loaded or stuff like signond will not work correctly with dbus-broker
• put the settings in a session.d file so we don't have to meddle with potential user settings (remove the old file if it contains our snippet)
• declare the doctype as required by the dbus-daemon documentation

(cherry picked from commit 09eabf34)