Ensure that the padding's statuslines are empty.

I modify buffer's statuslines via autocmds to indicate their state via my StatusLineHighlight plugin (vimscript #3383). Together with `:DistractFree`, that leaves a visible statusline in the padding window above the main window.
To avoid any such interference from autocmds, move the modification of the `&statusline` option in `s:SaveRestore()` to before the creation of the padding windows, and explicitly set the local statusline for those (just to be explicit and sure), too.