Accept new `always' value for option `buffer-offer-save'

Accept new `always' value for option `buffer-offer-save'

Also revert ee512e9a82

* lisp/files.el (buffer-offer-save): In addition to nil and t, now
  allows a third symbol value, `always'. A buffer where this option is
  set to `always' will always be offered for save by
  `save-some-buffers'.
  (save-some-buffers): Check the exact value of this buffer-local
  variable. No longer check the buffer name, or the value of
  `write-contents-functions'.
* doc/lispref/buffers.texi (Killing Buffers): Note change in manual.
* doc/lispref/files.texi (Saving Buffers): Remove note about buffer
  names.
* etc/NEWS: Mention in NEWS.