Ask if dir and subdir dired buffers be killed when deleting dir

Ask if dir and subdir dired buffers be killed when deleting dir

Previously, when you've had dired buffers

  ~/foo/
  ~/foo/bar/
  ~/foo/bar/baz/

and then deleted ~/foo/, dired (with dired-clean-up-buffers-too set to
non-nil) would only ask to delete the dired buffer of ~/foo/.  Now it
will offer to delete all three buffers.

* lisp/dired.el (dired-buffers-for-dir): Add optional argument SUBDIRS
which makes the function return also dired buffers showing a subdir of
DIR.
(dired-in-this-tree-p): Make obsolete in favor of file-in-directory-p
which actually does what the name suggest whereas dired-in-this-tree-p
is just string-matching on filenames which will fail with symlinks
filenames including ./ or ../.