Add new variable 'kill-buffer/delete-auto-save-files'

Add new variable 'kill-buffer/delete-auto-save-files'

* doc/emacs/files.texi (Auto Save Files): Document it.
* lisp/cus-start.el (standard): Add customize form.

* lisp/files.el (delete-auto-save-files): Move definition to C
(since it's used in the C layer).

* src/buffer.c (Fkill_buffer): Use the new variable (and remove
the old code that apparently didn't trigger for
kill-buffer/delete-auto-save-files.
(syms_of_buffer): Add new variable
kill-buffer-delete-auto-save-files and move definition of
delete-auto-save-files here (bug#21612).