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

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).