tui: Use underline cursor in Replace mode

This is a port of my original contribution to Vim, added in 7.4.687
(https://github.com/vim/vim/commit/v7-4-687). The TUI code has been
heavily refactored (see esp. 25ceadab37),
so this required some translation, but the logic is the same.

src/nvim/ui.c

@@ -476,7 +476,9 @@ static void ui_mode_change(void)
     return;
   }
   /* Get a simple UI mode out of State. */
-  if (State & INSERT)
+  if ((State & REPLACE) == REPLACE)
+    mode = REPLACE;
+  else if (State & INSERT)
     mode = INSERT;
   else
     mode = NORMAL;