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/20-dummy-headless.conf

@@ -0,0 +1,23 @@
+Section "Device"
+    Identifier "DummyHeadless"
+    Driver "dummy"
+    Option "ConstantDPI" "true"
+    VideoRam 256000
+EndSection
+
+Section "Monitor"
+    Identifier "DummyMonitor"
+    HorizSync 28-80
+    VertRefresh 48-75
+EndSection
+
+Section "Screen"
+    Identifier "DummyScreen"
+    Device "DummyHeadless"
+    Monitor "DummyMonitor"
+    DefaultDepth 24
+    SubSection "Display"
+        Depth 24
+        Modes "1920x1080" "1280x720"
+    EndSubSection
+EndSection