bug#46621: Copy line

[Helmut Eller]

> Please don't give this command a default key binding.
>
> There are a zillion commands that deserve a default
> binding more than this one does.  And nearly all of
> those zillion also don't need a default binding.

There is are default bindings for transpose-lines and for
count-lines-page.  Neither of those seems particularly useful.

Maybe there could be a prefix map for "line commands".  Something like
ctl-x-r-map for rectangle commands.  Perhaps ctl-x-l-map.

Helmut