bug#24880: 26.0.50; buff-menu: Command to unmark all buffers

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

It would be far better, IMHO, to take the approach used by
Dired for this. 

In Dired, `M-DEL' (or `* ?') unmarks a specific mark (e.g. D).
Or if it is followed directly by `RET', it unmarks all marks.

This is relevant and useful behavior for all contexts, such
as `*Buffer List*' (aka Buffer Menu), that let you mark
things in more than one way.

Users should be able to use `M-DEL' to selectively remove
any one of the marks (D, S, >,...) _or_ all marks together
(via `M-DEL RET').

The same approach should be taken for other, similar
buffers that list objects that can be marked in more than
one way, whether the objects be buffers (e.g. Ibuffer),
packages, files, bookmarks, or whatever else.

Besides giving users much more control, this provides a
consistent UI.  Users will then use `M-DEL RET' as the
standard way to completely unmark an object.

This is the approach I took with Bookmark+, for instance.
It takes a tiny bit more work to implement, but the result
is far more useful for users.

(Just one opinion.)