bug#42708: Let users copy "*Char Help*" buffer

[Juri Linkov]

>> 2. Add handling of 'help-char' to 'read-char-from-minibuffer',
>>    and leave 'read-char-choice' unchanged, then replace all calls
>>    of 'read-char-choice' with 'read-char-from-minibuffer' calls
>>    in the Emacs source tree.
>
> I'd go for 2, I think.  These functions are so similar in what they
> offer that it doesn't make much sense in having both.

So I pushed 2 to master.  Please check if everything is alright.
Hope this will make Drew happy.