bug#34715: 26.1; (1) Add `clone-frame', (2) bind it to `C-x 5 2'

[Eli Zaretskii]

> Date: Sat, 2 Mar 2019 16:47:47 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> 
> 3. BTW, I think it would be good to add this to the doc string of
>    `make-frame-command':
> 
>    Return the new frame.

Done.