bug#64185: proposal for new function: copy-line

[Mattias Engdegård]

>  'duplicate-dwim' duplicates the region if it is active.  If not, it
>  works like 'duplicate-line'.  An active rectangular region is
> -duplicated on its right-hand side.
> +duplicated on its right-hand side.  The new user option
> +'duplicate-line-final-position' specifies where to move point
> +after duplicating the line.

This makes it unclear to what extent the variable affects `duplicate-dwim`. 
(For some reason it only does so when the region is inactive, which doesn't 
seem right.)

And I agree that allowing any integer as value isn't necessary the best choice 
here. It's a false flexibility; nobody will ever set it to something other than 
0, 1 or -1. Symbols would probably be better, for several reasons.

Finally, it's a bit surprising that this is even discussed for inclusion in 
Emacs 29 at this stage, given the raw state of the design. What about we do a 
proper job on master instead, instead of rushing a half-baked new feature into 
a branch that is already deep into pre-release?