Merge branch 'jt/trace-error-on-warning'

Like die() and error(), a call to warning() will also trigger a
trace2 event.

* jt/trace-error-on-warning:
  usage: add trace2 entry upon warning()

usage.c

@@ -81,6 +81,12 @@ static void error_builtin(const char *err, va_list params)
 
 static void warn_builtin(const char *warn, va_list params)
 {
+	/*
+	 * We call this trace2 function first and expect it to va_copy 'params'
+	 * before using it (because an 'ap' can only be walked once).
+	 */
+	trace2_cmd_error_va(warn, params);
+
 	vreportf("warning: ", warn, params);
 }