fix: VPS xorg-dummy auth + rustdesk-dummy service fixes

- xorg-dummy.service: remove -nolisten local (blocked local UNIX socket)
- xorg-dummy.service: add -auth /root/.Xauthority and xauth generation ExecStartPre
- rustdesk-dummy.service: add XAUTHORITY=/root/.Xauthority env var
- Add config/vps/20-dummy-headless.conf (Xorg dummy config for VPS)
- Disable conflicting gdm.service on VPS
- RustDesk server now starts successfully with DUMMY0 as display

Fixes connection refused on /tmp/.X11-unix/X0 caused by
-nolisten local Xorg flag.

config/vps/rustdesk-dummy.service

@@ -1,5 +1,5 @@
 [Unit]
-Description=RustDesk Server with Dummy Display
+Description=RustDesk with Dummy Display
 After=xorg-dummy.service
 Requires=xorg-dummy.service
 
@@ -7,12 +7,11 @@ Requires=xorg-dummy.service
 Type=simple
 User=root
 Environment="DISPLAY=:0"
-ExecStartPre=/bin/sleep 5
+Environment="XAUTHORITY=/root/.Xauthority"
+ExecStartPre=/bin/sleep 3
 ExecStart=/usr/lib/rustdesk/rustdesk --server
 Restart=always
 RestartSec=10
-StandardOutput=append:/var/log/rdtop-rustdesk.log
-StandardError=append:/var/log/rdtop-rustdesk.log
 
 [Install]
 WantedBy=multi-user.target