Use the new "unclever" functionality of Home and End to make do_left()
and do_right() move properly to the end of the previous chunk or to the
start of the next chunk in softwrap mode when crossing a line boundary.
(Furthermore, doing Up plus End, or Down plus Home, does all needed
screen updates, which simplifies the code.)

The do_left() and do_right() functions don't yet properly move vertically
at line boundaries, but that will be fixed once do_up() and do_down() are
updated for softwrap mode, which is forthcoming.

This fixes https://savannah.gnu.org/bugs/?49384.