tcp: Refactor to use events instead of states, split out spliced implementation

Using events and flags instead of states makes the implementation
much more straightforward: actions are mostly centered on events
that occurred on the connection rather than states.

An example is given by the ESTABLISHED_SOCK_FIN_SENT and
FIN_WAIT_1_SOCK_FIN abominations: we don't actually care about
which side started closing the connection to handle closing of
connection halves.

Split out the spliced implementation, as it has very little in
common with the "regular" TCP path.

Refactor things here and there to improve clarity. Add helpers
to trace where resets and flag settings come from.

No functional changes intended.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

passt.c

@@ -119,12 +119,12 @@ static void post_handler(struct ctx *c, struct timespec *now)
 #define CALL_PROTO_HANDLER(c, now, lc, uc)				\
 	do {								\
 		extern void						\
-		lc ## _defer_handler (struct ctx *c)			\
+		lc ## _defer_handler (struct ctx *, struct timespec *)	\
 		__attribute__ ((weak));					\
 									\
 		if (!c->no_ ## lc) {					\
 			if (lc ## _defer_handler)			\
-				lc ## _defer_handler(c);		\
+				lc ## _defer_handler(c, now);		\
 									\
 			if (timespec_diff_ms((now), &c->lc.timer_run)	\
 			    >= uc ## _TIMER_INTERVAL) {			\