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/xorg-dummy.service

@@ -1,25 +1,16 @@
 [Unit]
-Description=Xorg Dummy Display for RustDesk VPS
+Description=Xorg Dummy Display
 After=network.target
 
 [Service]
 Type=simple
 User=root
 Environment="DISPLAY=:0"
+ExecStartPre=/bin/bash -c 'test -f /root/.Xauthority || (xauth -f /root/.Xauthority add :0 . $(mcookie) \u0026\u0026 chmod 600 /root/.Xauthority)'
 ExecStartPre=/bin/mkdir -p /tmp/.X11-unix
-ExecStart=/usr/lib/xorg/Xorg :0 \
-    -nolisten tcp \
-    -nolisten local \
-    -config /etc/X11/xorg.conf.d/99-dummy.conf \
-    +extension GLX \
-    +extension RANDR \
-    +extension RENDER \
-    -novtswitch \
-    -keeptty
+ExecStart=/usr/lib/xorg/Xorg :0     -auth /root/.Xauthority     -nolisten tcp     -config /etc/X11/xorg.conf.d/20-dummy-headless.conf     +extension GLX     +extension RANDR     +extension RENDER     -novtswitch     -keeptty
 Restart=always
 RestartSec=5
-StandardOutput=append:/var/log/rdtop-xorg.log
-StandardError=append:/var/log/rdtop-xorg.log
 
 [Install]
 WantedBy=multi-user.target