add check for display manager

install

@@ -13,7 +13,7 @@
 # https://github.com/armbian/config/blob/master/debian-software
 # https://forum.openmediavault.org/index.php/Thread/25062-Install-OMV5-on-Debian-10-Buster/
 #
-# version: 1.5.2
+# version: 1.6.0
 #
 
 if [[ $(id -u) -ne 0 ]]; then
@@ -27,6 +27,12 @@ if [ ! "${systemd}" = "systemd" ]; then
   exit 100
 fi
 
+if dpkg -l | grep -Eqw "gdm3|sddm|lxdm|xdm|lightdm|slim|wdm"; then
+  echo "This system is running a desktop environment!"
+  echo "This is not supported.  Exiting..."
+  exit 101
+fi
+
 declare -i cfg=0
 declare -i ipv6=0
 declare -i rpi=0