MINOR: actions: mutualise the action keyword lookup

Each (http|tcp)-(request|response) action use the same method
for looking up the action keyword during the cofiguration parsing.

This patch mutualize the code.
diff --git a/include/types/action.h b/include/types/action.h
index 9fb0878..b8b4cd4 100644
--- a/include/types/action.h
+++ b/include/types/action.h
@@ -1,6 +1,6 @@
 /*
  * include/types/action.h
- * This file contains TCP protocol definitions.
+ * This file contains actions definitions.
  *
  * Copyright (C) 2000-2010 Willy Tarreau - w@1wt.eu
  *