A user in the matrix.org Emacs chat room was asking why they couldn't
open multiple directories in multiple dired buffers, mark various files
in each of those buffers, and then perform some action on ALL of those
marked files at once.

My first thought was that this would be chaotic due to the possibility
of dired buffers with marked files which the user wasn't aware of at
the time; but then I remembered that you can pass `dired' a file list
in order to create a new dired buffer with only those files, and I
thought *that* sounded like a nice feature which addressed the
requirement in a safe manner (as the user then gets to review the file
list in the new dired buffer before acting on them).

I've attached a patch with a first pass implementation.

Thoughts?