GDBus: try XDG_RUNTIME_DIR/bus before resorting to dbus-launch

GDBus: try XDG_RUNTIME_DIR/bus before resorting to dbus-launch

This is the right thing to do for the "a session is a user-session"
model implemented in dbus 1.9.14, which is described in
<http://lists.freedesktop.org/archives/dbus/2015-January/016522.html>.

It also resembles sd-bus' behaviour, although sd-bus will only try
kdbus and XDG_RUNTIME_DIR/bus, and never runs dbus-launch.

On systems following the more traditional "a session is a login-session"
model, X_R_D/bus won't exist, so it is harmless to check for it before
falling back to X11 autolaunching. Again, this matches the behaviour
of current libdbus and sd-bus versions.

Now that we do this, g_test_dbus_unset() needs to clear XDG_RUNTIME_DIR
as well as everything else.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=747941
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>