[MINOR] term_trace: add better instrumentations to trace the code

A new member has been added to the struct session. It keeps a trace
of what block of code performs a close or a shutdown on a socket, and
in what sequence. This is extremely convenient for post-mortem analysis
where flag combinations and states seem impossible. A new ABORT_NOW()
macro has also been added to make the code immediately segfault where
called.
diff --git a/src/proto_uxst.c b/src/proto_uxst.c
index 9053805..f8026ce 100644
--- a/src/proto_uxst.c
+++ b/src/proto_uxst.c
@@ -413,6 +413,7 @@
 
 		s->flags = 0;
 		s->analysis = 0;
+		s->term_trace = 0;
 
 		if ((t = pool_alloc2(pool2_task)) == NULL) {
 			Alert("out of memory in uxst_event_accept().\n");