bug#28412: 26.0.50; Let save-some-buffers accept write-contents-functions

[Eric Abrahamsen]

This is about letting `save-some-buffers' check
`write-contents-functions' before insisting on a buffer having a file to
write to.

There was a conversations about this on emacs.devel:

https://lists.gnu.org/archive/html/emacs-devel/2017-05/msg00653.html

The patch I ended up with is attached.

I'm reporting this just so the patch (and the idea) doesn't get lost. I
still think it's worth doing.

0001-First-whack-at-write-contents-functions-for-non-file.patch
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