Switch user is broken for multiple sessions after upgrade to stretch.

[Sean Peters <sean.a.peters@googlemail.com>]

I've got a home desktop PC that often has several members of the
family logged in to it at the same time, with a mixture of LXDE,
GNOME, and FVWM(from .xsession) sessions.
It uses gdm3 as a display manager.

Since an upgrade to stretch, Switch User doesn't work.
None of these work:
  - "Switch User" option in the LXDE logout menu.
  - "New Login" button in xscreensaver,
  - "Switch User" button in gnome-screensaver,
  - gdmflexiserver
(It does work in GNOME)

I've also tried other display managers - LDXM, lightdm, sddm, wdm,
xdm - each has a switch user problem with some of the desktop
sessions.  Also, only gmd3 seems to remember the session choices on a
per user basis.

Any suggestions?

(There is also a problem that LXDE uses light-locker, which, when used
 with gdm3, just gives a blank screen with no way to get out of it.
 I've disabled light-locker for LXDE in
 /etc/xdg/autostart/light-locker.desktop to get around this.)

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