SSH port forwarding! How cool is that?

Only currently works on SSH1; SSH2 should be doable but it's late
and I have other things to do tonight. The Cool Guy award for this
one goes to Nicolas Barry, for doing most of the work and actually
understanding the code he was adding to.

[originally from svn r1176]

network.h

@@ -48,6 +48,10 @@ struct plug_function_table {
      *  - urgent==2. `data' points to `len' bytes of data,
      *    the first of which was the one at the Urgent mark.
      */
+    int (*accepting)(Plug p, struct sockaddr *addr, void *sock);
+    /*
+     * returns 0 if the host at address addr is a valid host for connecting or error
+     */
 };
 
 
@@ -59,6 +63,10 @@ void sk_addr_free(SockAddr addr);
 Socket sk_new(SockAddr addr, int port, int privport, int oobinline,
 	      Plug p);
 
+Socket sk_newlistenner(int port, Plug plug);
+
+Socket sk_register(void *sock, Plug plug);
+
 #define sk_plug(s,p) (((*s)->plug) (s, p))
 #define sk_close(s) (((*s)->close) (s))
 #define sk_write(s,buf,len) (((*s)->write) (s, buf, len))
@@ -68,6 +76,7 @@ Socket sk_new(SockAddr addr, int port, int privport, int oobinline,
 #ifdef DEFINE_PLUG_METHOD_MACROS
 #define plug_closing(p,msg,code,callback) (((*p)->closing) (p, msg, code, callback))
 #define plug_receive(p,urgent,buf,len) (((*p)->receive) (p, urgent, buf, len))
+#define plug_accepting(p, addr, sock) (((*p)->accepting)(p, addr, sock))
 #endif
 
 /*
@@ -88,6 +97,14 @@ void *sk_get_private_ptr(Socket s);
 char *sk_addr_error(SockAddr addr);
 #define sk_socket_error(s) (((*s)->socket_error) (s))
 
+/*
+ * Set the `frozen' flag on a socket. A frozen socket is one in
+ * which all sends are buffered and receives are ignored. This is
+ * so that (for example) a new port-forwarding can sit in limbo
+ * until its associated SSH channel is ready, and then pending data
+ * can be sent on.
+ */
+void sk_set_frozen(Socket sock, int is_frozen);
 
 /********** SSL stuff **********/
 
@@ -96,7 +113,6 @@ char *sk_addr_error(SockAddr addr);
  * of what it will eventually look like.
  */
 
-
 typedef struct certificate *Certificate;
 typedef struct our_certificate *Our_Certificate;
     /* to be defined somewhere else, somehow */