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/testdir/test49.in b/src/testdir/test49.in
index 79f13f6..a0c9e0d 100644
--- a/src/testdir/test49.in
+++ b/src/testdir/test49.in
@@ -5,7 +5,7 @@
 
 STARTTEST
 :so small.vim
-:se nocp nomore viminfo+=nviminfo
+:se nocp nomore viminfo+=nviminfo modelineexpr
 :lang mess C
 :so test49.vim
 :" Go back to this file and append the results from register r.