patch 8.1.0538: evaluating a modeline might invoke using a shell command

Problem:    Evaluating a modeline might invoke using a shell command. (Paul
            Huber)
Solution:   Set the sandbox flag when setting options from a modeline.

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    538,
 /**/
     537,
 /**/