vty/command: restrict the use of '.', '!', and '@' as flags

Change-Id: Icb4acbab0a15de2b0ed7b88fb0e227675317146a
Related: SYS#4937
diff --git a/tests/vty/vty_test.err b/tests/vty/vty_test.err
index a85b129..1cb4190 100644
--- a/tests/vty/vty_test.err
+++ b/tests/vty/vty_test.err
@@ -1,6 +1,9 @@
 Found duplicate flag letter 'f' in application specific attributes (index 0 vs 3)! Please fix.
 Found duplicate flag letter 'z' in application specific attributes (index 2 vs 4)! Please fix.
 Attribute flag letter 'X' is reserved for libraries! Please fix.
+Attribute flag character '.' is reserved for globals! Please fix.
+Attribute flag character '!' is reserved for globals! Please fix.
+Attribute flag character '@' is reserved for globals! Please fix.
 Got VTY event: 2
 Got VTY event: 2
 Got VTY event: 2