Make querying to kill processes customizable

Make querying to kill processes customizable

Introduce a new customization option, `confirm-kill-processes', that
users can set to nil if they don't want Emacs to nag them about killing
processes.

* lisp/files.el (confirm-kill-processes): New customization option.
(save-buffers-kill-emacs): Use customization option.

* test/lisp/files-tests.el
(files-test--save-buffers-kill-emacs--confirm-kill-processes): Add
test for new customization option.

* doc/emacs/entering.texi (Exiting): Document new user option.

* doc/lispref/processes.texi (Query Before Exit): Document new
user option.

* etc/NEWS: Document new user option.