[MEDIUM] make the http server error function a pointer in the session

It was a bit awkward to have session.c call return_srv_error() for
HTTP error messages related to servers. The function has been adapted
to be passed a pointer to the faulty stream interface, and is now a
pointer in the session. It is possible that in the future, it will
become a callback in the stream interface itself.
diff --git a/src/client.c b/src/client.c
index bc7b254..bf7f55c 100644
--- a/src/client.c
+++ b/src/client.c
@@ -212,6 +212,11 @@
 		else
 			s->do_log = tcp_sess_log;
 
+		if (p->mode == PR_MODE_HTTP)
+			s->srv_error = http_return_srv_error;
+		else
+			s->srv_error = NULL;
+
 		s->logs.accept_date = date; /* user-visible date for logging */
 		s->logs.tv_accept = now;  /* corrected date for internal use */
 		tv_zero(&s->logs.tv_request);