Add support for Windows named pipes.

This commit adds two new support modules, winnpc.c and winnps.c, which
deal respectively with being a client and server of a Windows named
pipe (which, in spite of what Unix programmers will infer from that
name, is actually closer to Windows's analogue of a Unix-domain
socket). Each one provides a fully featured Socket wrapper around the
hairy Windows named pipe API, so that the rest of the code base should
be able to use these interchangeably with ordinary sockets and hardly
notice the difference.

As part of this work, I've introduced a mechanism in winhandl.c to
permit it to store handles of event objects on behalf of other Windows
support modules and deal with passing them to applications' main event
loops as necessary. (Perhaps it would have been cleaner to split
winhandl.c into an event-object tracking layer analogous to uxsel, and
the handle management which is winhandl.c's proper job, but this is
less disruptive for the present.)

[originally from svn r10069]

windows/winstuff.h

@@ -501,6 +501,8 @@ void handle_got_event(HANDLE event);
 void handle_unthrottle(struct handle *h, int backlog);
 int handle_backlog(struct handle *h);
 void *handle_get_privdata(struct handle *h);
+struct handle *handle_add_foreign_event(HANDLE event,
+                                        void (*callback)(void *), void *ctx);
 
 /*
  * winpgntc.c needs to schedule callbacks for asynchronous agent