[MEDIUM] add support for "show sess" in unix stats socket

It is now possible to list all known sessions by issuing "show sess"
on the unix stats socket. The format is not much evolved but it is
very useful for debugging.

The doc has been updated to reflect the new keyword.
diff --git a/include/types/session.h b/include/types/session.h
index 7630bc3..fb63347 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -201,6 +201,9 @@
 			unsigned int flags;	/* STAT_* */
 			int iid, type, sid;	/* proxy id, type and service id if bounding of stats is enabled */
 		} stats;
+		struct {
+			struct bref bref;
+		} sess;
 	} data_ctx;				/* used by produce_content to dump the stats right now */
 	unsigned int uniq_id;			/* unique ID used for the traces */
 };