Description of problem:
Lightdm doesn't start X server when xserver-allow-tcp=true is present in [SeatDefaults] section of /etc/lightdm/lightdm.conf

Version-Release number of selected component (if applicable):
lightdm-1.10.5-3.el7.x86_64
lightdm-gtk-common-1.8.5-19.el7.noarch
lightdm-gobject-1.10.5-3.el7.x86_64
lightdm-gtk-1.8.5-19.el7.x86_64

How reproducible:
Always

Steps to Reproduce:
1. Set xserver-allow-tcp=true in [SeatDefaults] section of /etc/lightdm/lightdm.conf
2. systemctl restart lightdm
3. X server does not start

Actual results:
No graphical login. Systemd reports service is disabled after continued failing start attempts.

Expected results:
Login screen displayed.

Additional info:
lightdm log file shows that in the above scenario it tries to launch X server with "-listen tcp" command line option. Such an option IS NOT available in Xserver (contrary to -nolisten option which is available and works as expected).