patch 9.0.0411: only created files can be cleaned up with one call

Problem:    Only created files can be cleaned up with one call.
Solution:   Add flags to mkdir() to delete with a deferred function.
            Expand the writefile() name to a full path to handle changing
            directory.

src/userfunc.c

@@ -5649,6 +5649,21 @@ ex_defer_inner(
     return add_defer(name, argcount, argvars);
 }
 
+/*
+ * Return TRUE if currently inside a function call.
+ * Give an error message and return FALSE when not.
+ */
+    int
+can_add_defer(void)
+{
+    if (!in_def_function() && get_current_funccal() == NULL)
+    {
+	semsg(_(e_str_not_inside_function), "defer");
+	return FALSE;
+    }
+    return TRUE;
+}
+
 /*
  * Add a deferred call for "name" with arguments "argvars[argcount]".
  * Consumes "argvars[]".