Revamp of the local X11 connection code. We now parse X display

strings more rigorously, and then we look up the local X authority
data in .Xauthority _ourself_ rather than delegating to an external
xauth program. This is (negligibly) more efficient on Unix, assuming
I haven't got it wrong in some subtle way, but its major benefit is
that we can now support X authority lookups on Windows as well
provided the user points us at an appropriate X authority file in
the standard format. A new Windows-specific config option has been
added for this purpose.

[originally from svn r8305]

ssh.h

@@ -2,6 +2,7 @@
 #include <string.h>
 
 #include "puttymem.h"
+#include "tree234.h"
 #include "network.h"
 #include "int64.h"
 #include "misc.h"
@@ -327,28 +328,85 @@ extern void pfd_unthrottle(Socket s);
 extern void pfd_override_throttle(Socket s, int enable);
 
 /* Exports from x11fwd.c */
-extern const char *x11_init(Socket *, char *, void *, void *, const char *,
-			    int, const Config *);
+enum {
+    X11_TRANS_IPV4 = 0, X11_TRANS_IPV6 = 6, X11_TRANS_UNIX = 256
+};
+struct X11Display {
+    /* Broken-down components of the display name itself */
+    int unixdomain;
+    char *hostname;
+    int displaynum;
+    int screennum;
+    /* OSX sometimes replaces all the above with a full Unix-socket pathname */
+    char *unixsocketpath;
+
+    /* PuTTY networking SockAddr to connect to the display, and associated
+     * gubbins */
+    SockAddr addr;
+    int port;
+    char *realhost;
+
+    /* Auth details we invented for the virtual display on the SSH server. */
+    int remoteauthproto;
+    unsigned char *remoteauthdata;
+    int remoteauthdatalen;
+    char *remoteauthprotoname;
+    char *remoteauthdatastring;
+
+    /* Our local auth details for talking to the real X display. */
+    int localauthproto;
+    unsigned char *localauthdata;
+    int localauthdatalen;
+
+    /*
+     * Used inside x11fwd.c to remember recently seen
+     * XDM-AUTHORIZATION-1 strings, to avoid replay attacks.
+     */
+    tree234 *xdmseen;
+};
+/*
+ * x11_setup_display() parses the display variable and fills in an
+ * X11Display structure. Some remote auth details are invented;
+ * the supplied authtype parameter configures the preferred
+ * authorisation protocol to use at the remote end. The local auth
+ * details are looked up by calling platform_get_x11_auth.
+ */
+extern struct X11Display *x11_setup_display(char *display, int authtype,
+					    const Config *);
+void x11_free_display(struct X11Display *disp);
+extern const char *x11_init(Socket *, struct X11Display *, void *,
+			    const char *, int, const Config *);
 extern void x11_close(Socket);
 extern int x11_send(Socket, char *, int);
-extern void *x11_invent_auth(char *, int, char *, int, int);
-extern void x11_free_auth(void *);
 extern void x11_unthrottle(Socket s);
 extern void x11_override_throttle(Socket s, int enable);
-extern int x11_get_screen_number(char *display);
-void x11_get_real_auth(void *authv, char *display);
 char *x11_display(const char *display);
-
 /* Platform-dependent X11 functions */
-extern void platform_get_x11_auth(char *display, int *proto,
-                                  unsigned char *data, int *datalen);
-extern const char platform_x11_best_transport[];
-/* best X11 hostname for this platform if none specified */
-SockAddr platform_get_x11_unix_address(const char *display, int displaynum,
-				       char **canonicalname);
-/* make up a SockAddr naming the address for displaynum */
+extern void platform_get_x11_auth(struct X11Display *display,
+				  const Config *);
+    /* examine a mostly-filled-in X11Display and fill in localauth* */
+extern const int platform_uses_x11_unix_by_default;
+    /* choose default X transport in the absence of a specified one */
+SockAddr platform_get_x11_unix_address(const char *path, int displaynum);
+    /* make up a SockAddr naming the address for displaynum */
 char *platform_get_x_display(void);
-/* allocated local X display string, if any */
+    /* allocated local X display string, if any */
+/* Callbacks in x11.c usable _by_ platform X11 functions */
+/*
+ * This function does the job of platform_get_x11_auth, provided
+ * it is told where to find a normally formatted .Xauthority file:
+ * it opens that file, parses it to find an auth record which
+ * matches the display details in "display", and fills in the
+ * localauth fields.
+ *
+ * It is expected that most implementations of
+ * platform_get_x11_auth() will work by finding their system's
+ * .Xauthority file, adjusting the display details if necessary
+ * for local oddities like Unix-domain socket transport, and
+ * calling this function to do the rest of the work.
+ */
+void x11_get_auth_from_authfile(struct X11Display *display,
+				const char *authfilename);
 
 Bignum copybn(Bignum b);
 Bignum bn_power_2(int n);