patch 8.1.1113: making an autocommand trigger once is not so easy

Problem:    Making an autocommand trigger once is not so easy.
Solution:   Add the ++once argument.  Also add ++nested as an alias for
            "nested". (Justin M. Keyes, closes #4100)
diff --git a/src/globals.h b/src/globals.h
index e7ea84e..adf98f2 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1401,6 +1401,7 @@
 EXTERN char e_invaddr[]	INIT(= N_("E14: Invalid address"));
 EXTERN char e_invarg[]	INIT(= N_("E474: Invalid argument"));
 EXTERN char e_invarg2[]	INIT(= N_("E475: Invalid argument: %s"));
+EXTERN char e_duparg2[]	INIT(= N_("E983: Duplicate argument: %s"));
 EXTERN char e_invargval[]	INIT(= N_("E475: Invalid value for argument %s"));
 EXTERN char e_invargNval[]	INIT(= N_("E475: Invalid value for argument %s: %s"));
 #ifdef FEAT_EVAL