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/ux_x11.c

@@ -119,5 +119,3 @@ void platform_get_x11_auth(char *display, int *protocol,
     pclose(fp);
     sfree(localbuf);
 }
-
-const char platform_x11_best_transport[] = "unix";