implemented IKEv2 force_encap connection parameter

enforces UDP encapsulation by faking NAT detection payloads
  to hurdle restrictive firewalls

src/stroke/stroke.h

@@ -183,6 +183,7 @@ struct stroke_msg_t {
 			int eap_type;
 			int mode;
 			int mobike;
+			int force_encap;
 			struct {
 				char *ike;
 				char *esp;