Created a shiny new abstraction for the socket handling. Has many

advantages:
 - protocol modules can call sk_write() without having to worry
   about writes blocking, because blocking writes are handled in the
   abstraction layer and retried later.
 - `Lost connection while sending' is a thing of the past.
 - <winsock.h> is no longer needed in most modules, because
   "putty.h" doesn't have to declare `SOCKET' variables any more,
   only the abstracted `Socket' type.
 - select()-equivalent between multiple sockets will now be handled
   sensibly, which opens the way for things like SSH port
   forwarding.

[originally from svn r745]

network.h

@@ -0,0 +1,44 @@
+/*
+ * Networking abstraction in PuTTY.
+ *
+ * The way this works is: a back end can choose to open any number
+ * of sockets - including zero, which might be necessary in some.
+ * It can register a function to be called when data comes in on
+ * any given one, and it can call the networking abstraction to
+ * send data without having to worry about blocking. The stuff
+ * behind the abstraction takes care of selects and nonblocking
+ * writes and all that sort of painful gubbins.
+ * 
+ * If urgent data comes in on a socket, the back end will read and
+ * discard up to the urgent pointer, then read the urgent byte and
+ * send _that_ to the receiver function with `urgent' set.
+ */
+
+typedef struct Socket_tag *Socket;
+typedef struct SockAddr_tag *SockAddr;
+typedef int (*sk_receiver_t)(Socket s, int urgent, char *data, int len);
+
+void sk_init(void);		       /* called once at program startup */
+
+SockAddr sk_namelookup(char *host, char **canonicalname);
+void sk_addr_free(SockAddr addr);
+
+Socket sk_new(SockAddr addr, int port, sk_receiver_t receiver);
+void sk_close(Socket s);
+void sk_write(Socket s, char *buf, int len);
+void sk_write_oob(Socket s, char *buf, int len);
+
+/*
+ * Each socket abstraction contains a `void *' private field in
+ * which the client can keep state.
+ */
+void sk_set_private_ptr(Socket s, void *ptr);
+void *sk_get_private_ptr(Socket s);
+
+/*
+ * Special error values are returned from sk_namelookup and sk_new
+ * if there's a problem. These functions extract an error message,
+ * or return NULL if there's no problem.
+ */
+char *sk_addr_error(SockAddr addr);
+char *sk_socket_error(Socket addr);