Before becoming the selection owner for _NET_WM_CM_Sn, compton will now check if
that selection is already owned (which means that another composite manager is
already running). If this check fails, startup will be refused. This behaviour
is required by EWMH / ICCCM.
Because this should catch all composite managers, the error message that was
used before when another manager is already running is reworded to mention that
the other manager does not follow EWMH.
Signed-off-by: Uli Schlachter <psychon@znc.in>
