allow printing completions vertically

[jidanni]

We read
       print-completions-horizontally (Off)
              If  set  to  On,  readline will display completions with matches
              sorted horizontally in alphabetical order, rather than down  the
              screen.
Maybe the above variable is mainly talking about sorting order, in which
case it is unfortunately named... because what I want is upon me typing:
$ ls /var/cache/apt/archives/xdm_1%3a1.1.<TAB>
instead of the current:
xdm_1%3a1.1.8-6_i386.deb  xdm_1%3a1.1.9-1_i386.deb  xdm_1%3a1.1.9-2_i386.deb
I want to see:
xdm_1%3a1.1.8-6_i386.deb
xdm_1%3a1.1.9-1_i386.deb
xdm_1%3a1.1.9-2_i386.deb
Which you will have to admit is 10 times easier on the eyes and brain to
sort out. Yes, there are just three matches in this case.

We see
       COLUMNS
              Used  by  the  select  builtin command to determine the terminal
              width when printing selection  lists.   Automatically  set  upon
              receipt of a SIGWINCH.
says it only is used by select, not what we're interested in. And even if
it was, we don't want to affect other programs too with COLUMNS (which we
export COLUMNS=80 mostly.)