Hi!

This new command is inspired by `dired-copy-filename-as-kill', but has a
few differences, that can be easily reversed if there is such need.

Changes:
1. even single filename is quoted;
2. filenames are relative by default, prefix makes them absolute;
3. value of prefix argument does not matter, in dired it has to be 0.

I bound it to the `w' key because `w' is currently unbound in
vc-dir-mode-map and it is what is used in dired.