bug#15746: 24.3; [PATCH] bookmark should confirm when overwrite

[Drew Adams <drew.adams@oracle.com>]

> How 'bout the following:
> - change bookmark-set to emit a clear message when it updates an
>   existing message, as well as recording the previous value
>   somewhere.
> - provide a bookmark-undo command so the user can undo his
>   bookmark-set when he discovers that he has just changed an
>   existing bookmark by mistake.
> This way, we avoid prompting Drew annoyingly when he knows full well
> he's updating the bookmark.
> Leo, would that be sufficient to avoid the regrets, or did you
> realize too late for a bookmark-undo to be of any use?

To quote a famous person ;-), that would really be overthinking this.

The simple solution, which I expect everyone might be able to agree
to, is to have two different commands.  And to bind them both to keys,
at least on `bookmark-map'.  Choose the default behavior you like for
the traditional key, `C-x r m'.  For the other command use, e.g.,
`C-x r M'.  Not a big deal.  Users can get either behavior at any time.

Update `bookmark-set' to check not only the case in question (same
name as existing bookmark, and no prefix arg) but also a global
variable.  Define the new command by let-binding that variable around
`call-interactively' of `bookmark-set'.