Windows Pageant: establish a named-pipe server.

This reuses all the named-pipe IPC code I set up for connection
sharing a few years ago, to set up a named pipe with a predictable
name and speak the stream-oriented SSH agent protocol over it.

In this commit, we just set up the server, and there's no code that
speaks the client end of the new IPC yet. But my plan is that clients
should switch over to using this interface if possible, because it's
generally better: it doesn't have to be handled synchronously in the
middle of a GUI event loop (either in Pageant itself _or_ in its
client), and it's a better fit to the connection-oriented nature of
forwarded agent connections (so if any features ever appear in the
agent protocol that require state within a connection, we'll now be
able to support them).

windows/winstuff.h

@@ -653,6 +653,11 @@ struct handle_sink {
 };
 void handle_sink_init(handle_sink *sink, struct handle *h);
 
+/*
+ * Exports from winpgntc.c.
+ */
+char *agent_named_pipe_name(void);
+
 /*
  * winpgntc.c needs to schedule callbacks for asynchronous agent
  * requests. This has to be done differently in GUI and console, so