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.

doc/design.txt

@@ -197,10 +197,13 @@ Fixed header
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           Version                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |                            Flags                              |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           Length                              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
 Version     : 4 bytes       : message version
+Flags       : 4 bytes       : extra flags
 Length      : 4 bytes       : total message length