[LightDM] lightdm goes into tight loop trying to create greeter sessions and fills up logs

[Greg Klanderman]

Please see

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=958718

I am seeing lightdm run fine for a couple days, then while there is
still an active user session on seat-1, it decides it needs to create
a greeter session on seat-1.  That fails repeatedly, with it
attempting to create the greeter session 30-50 times per second.  This
quickly fills up the log file to many Gb, and uses significant CPU.

See the debian bug for details and log snippets.

thank you,
Greg