tinydm-run-session.sh: support starting dbus session
If a tinydm session is unable to start a dbus session automatically, e.g. i3wm, support starting one if configured in the profile. This is useful to run programs that require a dbus session bus, e.g. feedbackd under such desktop environments.
Reference: https://wiki.gentoo.org/wiki/Elogind#startx_D-Bus_integration