[displaymanager] Handle variations in GDM executable name

- Debian https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=934504
 - Instead of hard-coding one or the other, search for gdm and gdm3

src/modules/displaymanager/main.py

@@ -249,6 +249,21 @@ class DMgdm(DisplayManager):
     name = "gdm"
     executable = "gdm"
 
+    def have_dm(self):
+        """
+        GDM exists with different executable names, so search
+        for one of them and use it.
+        """
+        for executable in ( "gdm", "gdm3" ):
+            bin_path = "{!s}/usr/bin/{!s}".format(self.root_mount_point, executable)
+            sbin_path = "{!s}/usr/sbin/{!s}".format(self.root_mount_point, executable)
+            if os.path.exists(bin_path) or os.path.exists(sbin_path):
+                # Keep the found-executable name around for later
+                self.executable = executable
+                return True
+
+        return False
+
     def set_autologin(self, username, do_autologin, default_desktop_environment):
         # Systems with GDM as Desktop Manager
         gdm_conf_path = os.path.join(self.root_mount_point, "etc/gdm/custom.conf")