dbus-run-session: remove various extra variables from the environment

dbus-run-session: remove various extra variables from the environment

DBUS_SESSION_BUS_PID is not mandatory to set, but we should unset it
if present, since it points to a different session's bus. Likewise for
DBUS_SESSION_BUS_WINDOWID.

Similarly, if DBUS_STARTER_BUS_TYPE and DBUS_STARTER_ADDRESS
are set (as they would be under GNOME Terminal 3.8, see
<https://bugs.freedesktop.org/show_bug.cgi?id=63119>) then they
are likely to point to a different session's bus.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=39196
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Colin Walters <walters@verbum.org>