Add option to manually specify XAuthority files for login greeters

[ehfd]

This is required because several login greeters specify their own XAuth files for MIT-MAGIC-COOKIE-1 authentications.

Example (systemctl status sddm):

● sddm.service - Simple Desktop Display Manager
     Loaded: loaded (/lib/systemd/system/sddm.service; enabled; vendor preset: enabled)
     Active: active (running) since Tue 2023-01-01 00:00:00 PST; 00min ago
       Docs: man:sddm(1)
             man:sddm.conf(5)
   Main PID: 000 (sddm)
      Tasks: 4 (limit: 38138)
     Memory: 00.0M
        CPU: 000ms
     CGroup: /system.slice/sddm.service
             ├─968 /usr/bin/sddm
             └─972 /usr/lib/xorg/Xorg -nolisten tcp -auth /var/run/sddm/{(hexadecimal strings)}

(I tried Selkies on a rebooted desktop and found this issue.)

I need to add a command-line option to specify this file to Selkies so that it doesn't show the MIT-MAGIC-COOKIE-1 error.

[ehfd]

I believe that the XAUTHORITY environment variable does this. Need to add to the docs.

[ehfd]

xauth list is another way to do it.

[ehfd]

Added docs.