Reinstate auto-added (y or n) suffix.
Added mode for asking among multiple options.

This allows you to ask, for example:

  Buffer foo is modified, save? (y, n, q, !, ., C-g, C-h)

The C-h is added automatically and displays help for the options.
Provide Buffer instead of BufferV.

We need to be able to save dirty buffers when exiting, and that's in Buffer,
not BufferV.
Wired up dirty buffer save in kill-editor.

We don't yet support diffing the buffer against the file system, but we'll get
that in there eventually.