use xauth in /usr/bin

Since the x.org rework, X has been installed into standard paths and not
its own random prefixes.  I think it's time we update the default paths
accordingly.

options.h

@@ -264,7 +264,7 @@ much traffic. */
 /* The command to invoke for xauth when using X11 forwarding.
  * "-q" for quiet */
 #ifndef XAUTH_COMMAND
-#define XAUTH_COMMAND "/usr/bin/X11/xauth -q"
+#define XAUTH_COMMAND "/usr/bin/xauth -q"
 #endif
 /* if you want to enable running an sftp server (such as the one included with