dbus proxy: fork a subprocess in some configurations

We do this when we service a multiplex downstream socket, and
demux that into separate upstream connections to DBus-daemon.
This is the configuration when running the proxy on the master
host to manage KVM guests, for instance.

This allows us to start only one DBus proxy process for all guests,
rather than having to take care of starting and stopping individual
processes for each guest.

Signed-off-by: Olaf Kirch <okir@suse.de>
Use one global dbus-proxy instance rather than a separate one for each guest

Signed-off-by: Olaf Kirch <okir@suse.de>
testbus-run: support topology files

Instead of creating instances from within the .run script, read the list
of instances needed for this test from a separate .conf file and instantiate
them.

Signed-off-by: Olaf Kirch <okir@suse.de>
dbus-proxy: prevent creation of zombies when child processes exit

Signed-off-by: Olaf Kirch <okir@suse.de>