Using do_up() and do_end() when the user types <Left> at the start of
a line, and do_down() and do_home() when typing <Right> at line's end
can be problematic when tabs are wider than the screen, because those
functions convert indexes to columns and back again twice, thus causing
inaccuracies.  Therefore, simply adjust current and current_x directly,
and then redraw the screen.

This fixes https://savannah.gnu.org/bugs/index.php?51778.