Exclusive window manager for -multiwindow

[Takuma Murakami]

I have committed a change for -multiwindow window manager
to the xorg tree.  Now the wm should be able to detect
another wm and the others can detect it.  This change
corresponds to the second item (Window manager detection)
in the To-Do list.

To do so the internal wm grabs ButtonPressMask, which is
allowed only one client at once.  Thus other wm's will fail
to obtain the mask and immediately exit when invoked.

Takuma Murakami