[MEDIUM] Collect & provide separate statistics for sockets, v2

This patch allows to collect & provide separate statistics for each socket.
It can be very useful if you would like to distinguish between traffic
generate by local and remote users or between different types of remote
clients (peerings, domestic, foreign).

Currently no "Session rate" is supported, but adding it should be possible
if we found it useful.
diff --git a/include/types/session.h b/include/types/session.h
index d3dbe67..356f0ca 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -196,6 +196,7 @@
 		struct {
 			struct proxy *px;
 			struct server *sv;
+			struct listener *l;
 			short px_st, sv_st;	/* DATA_ST_INIT or DATA_ST_DATA */
 			unsigned int flags;	/* STAT_* */
 			int iid, type, sid;	/* proxy id, type and service id if bounding of stats is enabled */