Allow extending 'save-some-buffers'

Allow extending 'save-some-buffers'

* lisp/abbrev.el (abbrev--possibly-save): Separated out from
`save-some-buffers'.
(save-some-buffers-functions): Add to the save function.

* lisp/files.el (save-some-buffers-functions): New variable.
(save-some-buffers): Use it.
(save-buffers-kill-emacs): Also use it to see if we have something
to save (bug#55579).