The single instance mode currently tries to contact another VLC instance
through dbus in order to find out if one is running. With dbus activation,
it results to the dbus daemon launching a new VLC instance while the
first one blocks until the new one answers the call

This patch uses org.freedesktop.DBus.NameHasOwner instead in order to get
an answer from the dbus daemon without it launching a second instance.
(cherry picked from commit bcc158b7)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>