bug#9563: dired-copy-filename-as-kill doesn't copy file names to PRIMARY selection

[Juri Linkov]

The default behavior is dangerous now on GNU/Linux,
it may delete random files.

Steps to reproduce the problem:

1. emacs -Q
2. Open a shell script, mark a multi-line region with a few commands
   and copy it with `M-w'.
3. Forget it.
4. Open a Dired buffer.
5. Copy a file name with `M-0 w'.
6. Open xterm.
7. Paste the copied file name with `S-Insert'.

Instead of pasting the copied file name
it starts executing random commands,
i.e. if text copied at step 2 contains newlines,
it's submitted for execution to the shell.

The problem is that `M-w' puts copied text to the primary selection
while `M-0 w' in Dired does not.

I think `dired-copy-filename-as-kill' should do the same
as `deactivate-mark' does with (x-set-selection 'PRIMARY ...)