MINOR: stream-int: make the shutr/shutw functions void

This is to be more consistent with the other functions. The only
reason why these functions used to return a value was to let the
caller adjust polling by itself, but now their only callers were
the si_shutr()/si_shutw() inline functions. Now these functions
do not depend anymore on the connection.

These connection variant of these functions now call
conn_data_stop_recv()/conn_data_stop_send() before returning order
not to require a return code anymore. The applet version does not
need this at all.
diff --git a/include/types/stream_interface.h b/include/types/stream_interface.h
index 257a5d6..9f8afef 100644
--- a/include/types/stream_interface.h
+++ b/include/types/stream_interface.h
@@ -85,8 +85,8 @@
 	void (*update)(struct stream_interface *);  /* I/O update function */
 	void (*chk_rcv)(struct stream_interface *); /* chk_rcv function */
 	void (*chk_snd)(struct stream_interface *); /* chk_snd function */
-	int (*shutr)(struct stream_interface *);    /* shut read function */
-	int (*shutw)(struct stream_interface *);    /* shut write function */
+	void (*shutr)(struct stream_interface *);   /* shut read function */
+	void (*shutw)(struct stream_interface *);   /* shut write function */
 };
 
 /* A stream interface has 3 parts :