patch 8.1.1366: using expressions in a modeline is unsafe

Problem:    Using expressions in a modeline is unsafe.
Solution:   Disallow using expressions in a modeline, unless the
            'modelineexpr' option is set.  Update help, add more tests.
diff --git a/src/option.h b/src/option.h
index 7a10bb9..082ff9b 100644
--- a/src/option.h
+++ b/src/option.h
@@ -631,6 +631,7 @@
 #ifdef FEAT_SPELL
 EXTERN char_u	*p_msm;		/* 'mkspellmem' */
 #endif
+EXTERN long	p_mle;		/* 'modelineexpr' */
 EXTERN long	p_mls;		/* 'modelines' */
 EXTERN char_u	*p_mouse;	/* 'mouse' */
 #ifdef FEAT_GUI