Fixed op_down().

Now it's possible to move down to the last line when it
doesn't have a newline.
Toggle end-of-line visible marks with Ctrl-K (temporary functionality).
Revert "Fixed op_down()."

This reverts commit b4e661e1d3c01d954e40d086aa39592ae1d08090.
In op_down() allow moving to the last, non-eol-limited line.
Read one more char than w in qw_blk_row_next().
Minor tweaks to curses driver.
New option op_tab().
Rewritten op_tab() to write all spaces in one step (for undo).