bug#51883: 29.0.50; Command to get accidentally deleted frames back

[Gregory Heytings]

> Thanks for your work so far.
>
> I tested your patch quickly.  It works!

Thanks for your feedback!

> But I noticed that C-x 5 u without prefix arg errors.  This is probably 
> not intended.  I think you rather want to test the raw prefix arg with 
> `consp', not with `listp'.  But why not just use (interactive "p")?

This is intended, it's what Eli wanted, unless I misunderstood what he 
meant.  In a previous version of the patch C-x 5 u without prefix arg 
undeleted the last deleted frame.  Now an explicit numerical prefix 
argument between 1 and 16 is required.