bug#21578: 24.5; `ibuffer-copy-filename-as-kill' unconsistent with `dired-copy-filename-as-kill'

[Tino Calancha]

dired-copy-filename-as-kill doesnt make distinction between dir/files.

Considering that these functions names just differ in the prefix,
and the documentation strings looks almost the same,
consistenty reasons demand that they behave almost the same:
ibuffer-copy-filename-as-kill should return the (dired-default-directory)
for those buffers in dired mode.

It would be also natural (but maybe not very useful) if
ibuffer-copy-filename-as-kill behaves as its dired partner when we
have marked buffers but we pass a numerical argument !=0: my patches didnt 
took this in account.
By the way, my patch3 should replace:
?>
with
ibuffer-marked-char