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]

windows/winsftp.c

@@ -19,6 +19,12 @@ int get_userpass_input(prompts_t *p, unsigned char *in, int inlen)
     return ret;
 }
 
+void platform_get_x11_auth(struct X11Display *display, const Config *cfg)
+{
+    /* Do nothing, therefore no auth. */
+}
+const int platform_uses_x11_unix_by_default = TRUE;
+
 /* ----------------------------------------------------------------------
  * File access abstraction.
  */