Fix illegal set operation in FreeBSD section of user module.

system/user

@@ -745,7 +745,7 @@ class FreeBsdUser(User):
                 cmd.append('-G')
                 new_groups = groups
                 if self.append:
-                    new_groups.extend(current_groups)
+                    new_groups = groups | set(current_groups)
                 cmd.append(','.join(new_groups))
 
         # modify the user if cmd will do anything