REORG/MEDIUM: replace stream interface protocol functions by a proto pointer

The stream interface now makes use of the socket protocol pointer instead
of the direct functions.
diff --git a/include/proto/protocols.h b/include/proto/protocols.h
index 05567ea..cebe560 100644
--- a/include/proto/protocols.h
+++ b/include/proto/protocols.h
@@ -125,6 +125,8 @@
  */
 int protocol_enable_all(void);
 
+/* returns the protocol associated to family <family> or NULL if not found */
+struct protocol *protocol_by_family(int family);
 
 #endif /* _PROTO_PROTOCOLS_H */