bug#3646: "bookmark format upgrade is incorrect"

[Karl Fogel]

The actual situation is much, much worse than even this.

Because the recent format upgrade (to the "third" format) did not bump
`bookmark-file-format-version', ever since at least rev 1.114 (and
possibly before) bookmark.el has been potentially writing
`bookmark-alist' in a mixture of formats: some of the bookmarks in the
alist might be in format 2, and some might be in format 1.

I'm working on a fix.  It should be possible to detect 1 and migrate it
to 2 at the individual bookmark record level.