This version is capable of shutting down several other computers, when the login server shuts down.

It needs modified Xstartup and Xreset scripts in the XDM configuration directory. This voodoo is needed due to a problem with utmp logging with sessreg:
:0 cannot show up multiple times at once, but it would have to, because everything before ":" is stripped.

So we replace :0 by @<hostname>. This makes the entry unique to the host.
Change the "-l"-argument of "sessreg" from "$DISPLAY" to "@${DISPLAY%:*}", so 

sessreg -a -l $DISPLAY -u /var/run/utmp -x /etc/X11/kdm/Xservers $USER
would become 
sessreg -a -l @${DISPLAY%:*} -u /var/run/utmp -x /etc/X11/kdm/Xservers $USER
Note, that this modification drops the uniqueness of the display at a particular host. But in the normal case every host will have only one XDM-managed display.

If you have multiple displays on a host, then use "${DISPLAY#*:}@${DISPLAY%:*}" instead.

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