bug#46621: Copy line

[Juri Linkov]

>   > It's not realistic to change the meaning of the numeric arg to C-y.
>   > People already use the current meaning for decades.
>
> We could inquire of the users to see what they think about the issue.

Every once in a while the lack of such fundamental command sparks very
long discussions how to reinvent the wheel.  Some recent examples:

https://stackoverflow.com/questions/88399/how-do-i-duplicate-a-whole-line-in-emacs
https://stackoverflow.com/questions/88399/how-do-i-duplicate-a-whole-line-in-emacs?page=2
https://stackoverflow.com/questions/27129147/efficent-way-to-copy-and-paste-a-line
https://stackoverflow.com/questions/12132601/duplicating-the-current-line
https://emacs.stackexchange.com/questions/22705/quickly-duplicate-line-and-increase-number-for-lots-of-lines
https://old.reddit.com/r/emacs/comments/lkyadp/weekly_tipstricketc_thread/gnnniq3/
https://old.reddit.com/r/emacs/comments/jvpccf/weekly_tipstricketc_thread/gcr8h68/
https://old.reddit.com/r/emacs/comments/jwhr6g/batteries_included_with_emacs/gcqdmp9/
with questions like from the last:

  Why does emacs doesn't have a built-in ergonomic way to duplicate a line?
  Genuinely curious, seems like something fundamental.  Guess it is one of
  the reasons evil-mode has rise so much.  Any new comer will feel disgusted
  by having to do C-a C-space C-e M-w and more keystrokes to place it

The proposed tiny 8-line patch was intended to help people avoid
wasting their time on such trivial things.