CLEANUP: tcp rules: mention everywhere that tcp-conn rules are L4

This is in order to make integration of tcp-request-session cleaner :
- tcp_exec_req_rules() was renamed tcp_exec_l4_rules()
- LI_O_TCP_RULES was renamed LI_O_TCP_L4_RULES
  (LI_O_*'s horrible indent was also fixed and a provision was left
   for L5 rules).
diff --git a/src/proto_tcp.c b/src/proto_tcp.c
index c3c998e..0223b8e 100644
--- a/src/proto_tcp.c
+++ b/src/proto_tcp.c
@@ -1353,7 +1353,7 @@
  * matches or if no more rule matches. It can only use rules which don't need
  * any data. This only works on connection-based client-facing stream interfaces.
  */
-int tcp_exec_req_rules(struct session *sess)
+int tcp_exec_l4_rules(struct session *sess)
 {
 	struct act_rule *rule;
 	struct stksess *ts;
@@ -1881,10 +1881,12 @@
 	else {
 		struct action_kw *kw;
 		if (where & SMP_VAL_FE_CON_ACC) {
+			/* L4 */
 			kw = tcp_req_conn_action(args[arg]);
 			rule->kw = kw;
 			rule->from = ACT_F_TCP_REQ_CON;
 		} else {
+			/* L6 */
 			kw = tcp_req_cont_action(args[arg]);
 			rule->kw = kw;
 			rule->from = ACT_F_TCP_REQ_CNT;