launch/activate: start transient unit if no systemd unit is configured
launch/activate: start transient unit if no systemd unit is configured

dbus service files can contain an Exec line instead of a systemd unit,
though that is discouraged. Translate this into an automatically
generated transient unit and start it through systemd.

This differs from the dbus daemon which forks of the service itself.

Signed-off-by: Tom Gundersen <7c8224529a7c718e876cb2496c973509fde1e50e@jklm.no>

bus1/dbus-broker