vim:aucmd: Underline old cursor position

Underline old cursor position for better readability. Reusing
`TermCursor` made me confuse the current window many times.

My favorite solution would be an empty box around the position similar
to how terminals do it when focus is lost. But I fear that this is not
possible with a simple highlighting group.

.config/vim/vimrc.d/80-autocommands.vim

@@ -104,10 +104,10 @@ augroup highlight_current_word
 	au CursorMovedI * call HighlightCurrentWord()
 augroup END
 
-" When switching focus to another window, keep the cursor location highlighted.
+" When switching focus to another window, keep the cursor location underlined.
 function! HighlightOldCursorPos()
 	let w:cursor_pos_match_id = matchaddpos(
-		\ 'TermCursor',
+		\ 'Underlined',
 		\ [getcurpos()[1:2]])
 endfunction
 function! ClearOldCursorPos()