vim-patch:8.1.0539: cannot build without the sandbox

Problem:    Cannot build without the sandbox.
Solution:   Set the secure option instead of using the sandbox.  Also restrict
            the characters from 'spelllang' that are used for LANG.vim.
            (suggested by Yasuhiro Matsumoto)
82e8c92ebe

src/nvim/buffer.c

@@ -4945,11 +4945,11 @@ chk_modeline (
       save_SID = current_SID;
       current_SID = SID_MODELINE;
       // Make sure no risky things are executed as a side effect.
-      sandbox++;
+      ++secure;
 
       retval = do_set(s, OPT_MODELINE | OPT_LOCAL | flags);
 
-      sandbox--;
+      --secure;
       current_SID = save_SID;
       if (retval == FAIL)                     /* stop if error found */
         break;