Add debconf question for display-manager, based on gdm3

debian/cosmic-greeter.postinst

@@ -0,0 +1,49 @@
+#!/bin/sh
+
+set -e
+
+. /usr/share/debconf/confmodule
+
+THIS_PACKAGE=cosmic-greeter
+DEFAULT_DISPLAY_MANAGER_FILE=/etc/X11/default-display-manager
+
+# debconf is not a registry, so we only fiddle with the default file if 
+# the configure script requested an update
+if [ -e $DEFAULT_DISPLAY_MANAGER_FILE.debconf-update ]; then
+  rm -f $DEFAULT_DISPLAY_MANAGER_FILE.debconf-update
+  if db_get shared/default-x-display-manager; then
+    # workaround debconf passthru bug (#379198)
+    if [ -z "$RET" ]; then
+      RET="$THIS_PACKAGE"
+    fi
+    if [ "$THIS_PACKAGE" != "$RET" ]; then
+      echo "Please be sure to run \"dpkg --configure $RET\"."
+    fi
+    if db_get "$RET"/daemon_name; then
+      echo "$RET" > $DEFAULT_DISPLAY_MANAGER_FILE
+    fi
+  fi
+fi
+
+DEFAULT_SERVICE=/etc/systemd/system/display-manager.service
+# set default-display-manager systemd service link according to our config
+if [ "$1" = configure ] && [ -d /etc/systemd/system/ ]; then
+  if [ -e "$DEFAULT_DISPLAY_MANAGER_FILE" ]; then
+    SERVICE=/lib/systemd/system/$(basename $(cat "$DEFAULT_DISPLAY_MANAGER_FILE")).service
+    if [ -h "$DEFAULT_SERVICE" ] && [ $(readlink "$DEFAULT_SERVICE") = /dev/null ]; then
+      echo "Display manager service is masked" >&2
+    elif [ -e "$SERVICE" ]; then
+      ln -sf "$SERVICE" "$DEFAULT_SERVICE"
+    else
+      echo "WARNING: $SERVICE is the selected default display manager but does not exist" >&2
+      rm -f "$DEFAULT_SERVICE"
+    fi
+  else
+    rm -f "$DEFAULT_SERVICE"
+  fi
+fi
+
+# debconf hangs if cosmic-greeter gets started below without this
+db_stop || true
+
+#DEBHELPER#