* lisp/bookmark.el (bookmark-set-internal): New helper function to do
what `bookmark-set' used to do, but with more choices for overwrite
vs push, and with minor changes to the interactive prompt format.
(bookmark-set): Rewrite as wrapper around above.
If overwriting, inform the user of that in the prompt.
(bookmark-set-no-overwrite): New function, also done as wrapper.
Bind to "M" in `ctl-x-r-map' autoloads.
(bookmark-map): Similarly bind "M" here.