Cursor binding.

Show scrollbind or cursorbind in the statusline.

If cursorbind (introduced in Vim 7.3) is on show a horizontal arrow
indicator.
If scrollbind is on (and cursorbind isn't) show a vertical indicator.

This necessitates moving Has_Unicode() to the Vim 5 section and hence
removing the script prefix.

Set cursorbind automatically where supported if diff mode is on.