Rework how clients connect.

Add buffer flags. The idea is to make it possible to easily check when a
buffer contains control information that we need to parse to update the
port fields.
Make the client create remote nodes and ports and set up proxies for
them.
Make a port base class implementing most of the logic to pass buffers
locally and remotely.
Remove most code from stream.c, it's now in the port.
Make a portsink and portsrc that can write and read to/from any port. We
use these in the server to send and receive data.
Rework format negotiation. The final format is now sent in-line before
the data. The server will select a format on output ports.

pinos/client/pinos.h

@@ -43,4 +43,6 @@ gchar *pinos_client_name (void);
 void   pinos_fill_context_properties (PinosProperties *properties);
 void   pinos_fill_stream_properties  (PinosProperties *properties);
 
+PinosDirection pinos_direction_reverse (PinosDirection direction);
+
 #endif /* __PINOS_H__ */