don't consider restore_cursor as a public interface and while there rename it as restore_curs_x, as it describes better the behaviour of the function. I have to disable the buffer->force-redraw optimization, at least from now, given that cmd.c don't update curs_y anymore, and with that optimisation in place the cursor y position doesn't get updated upon redraw. Add a comment as a future note though.

instead of using the line_off + vline_nth, cache the current top line.

`attr' now follows the same rule as `fg' and `bg', with the three airity variant. Attributes by themselves are a comma-separated list of keywords, each one representing a ncurses attribute.