bug#27634: 25.2.1; C-g does not quit register-read-with-preview

[Tino Calancha]

On Tue, 11 Jul 2017, Paul Rankin wrote:

> One overlooked thing about Tino's solution is that C-g is a keystroke and keyboard-quit 
> is a function, which obviously aren't necessarily equivalent. What if the user remaps 
> keyboard quit to "7"?
I thought about that, but i discarded because i think binding something
other that `keyboard-quit' to `C-g' is a misuse.  The Emacs manual is full
of mentions to `C-g' as `keyboard-quit'.
There is even the following remark in the tips section:

"don't bind a key sequence ending in @key{C-g}, since that
is commonly used to cancel a key sequence."

If a user want to ignore such kind of advice he/she should
not expect everything will work the same.

Maybe we can fix this so that `register-read-with-preview'
will work with `C-g' bound to `my-cool-foo-command'; but we
cannot assure that no other Emacs part is affected because such
misguided `C-g' binding.  We must encourage users to follow
good practices.

Sure, it would be great if the entire Emacs code is robust against
any kind of user abuse/misuse, but that's not realistic.