[MEDIUM] make the global stats socket part of a frontend

Creating a frontend for the global stats socket will help merge
unix sockets management with the other socket management. Since
frontends are huge structs, we only allocate it if required.
diff --git a/src/proto_uxst.c b/src/proto_uxst.c
index 8c1f2c5..742d116 100644
--- a/src/proto_uxst.c
+++ b/src/proto_uxst.c
@@ -450,8 +450,7 @@
 
 		s->task = t;
 		s->listener = l;
-		s->fe = NULL;
-		s->be = NULL;
+		s->fe = s->be = l->private;
 
 		s->ana_state = 0;
 		s->req = s->rep = NULL; /* will be allocated later */