Hi, Eli and Dmitry!

I tried to write documentation for the previous version of the command
and came to the conclusion that I crammed too much functionality into
it.  So I divided the DWIM command into command for copying name at
point (bound to w) and command for copying marked files (bound to * w).

I also added new command for copying VC related filenames from
everywhere.  An appropriate binding would be C-x v w, but I am not sure
if I should use a top-level binding for such an insignificant command.

Quoting and prefix arguments now copy `dired-copy-filename-as-kill'.