X forwarding changes:

- new function platform_get_x_display() to find a sensible local display.
   On Unix, the Gtk apps weren't taking account of --display when
   determining where to send forwarded X traffic.
 - explicitly document that leaving X display location blank in config tries
   to do something sensible (and that it's now blank by default)
 - don't override X11Display setting in plink, since that's more properly
   done later

[originally from svn r4604]

unix/uxplink.c

@@ -73,8 +73,6 @@ static Config cfg;
  */
 char *platform_default_s(const char *name)
 {
-    if (!strcmp(name, "X11Display"))
-	return dupstr(getenv("DISPLAY"));
     if (!strcmp(name, "TermType"))
 	return dupstr(getenv("TERM"));
     if (!strcmp(name, "UserName"))