patch 8.1.0540: may evaluate insecure value when appending to option

Problem:    May evaluate insecure value when appending to option.
Solution:   Set the secure flag when changing an option that was previously
            set insecurely.  Also allow numbers for the characters from
            'spelllang' that are used for LANG.vim.
diff --git a/src/version.c b/src/version.c
index a6f155f..f3889b0 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    540,
+/**/
     539,
 /**/
     538,