MINOR: action: add private configuration

This private configuration pointer is used for storing some configuration
data associated the keyword, So many keywords can use the same parse
function, and this one can use a discriminator.

include/types/action.h

@@ -154,6 +154,7 @@ struct action_kw {
 	enum act_parse_ret (*parse)(const char **args, int *cur_arg, struct proxy *px,
 	                            struct act_rule *rule, char **err);
 	int match_pfx;
+	void *private;
 };
 
 struct action_kw_list {