[coreutils] stat: reverting recent %X %Y %Z change

[Jim Meyering]

Part of reverting this change:

    stat now outputs the full sub-second resolution for the atime,
    mtime, and ctime values since the Epoch, when using the %X, %Y, and
    %Z directives of the --format option.  This matches the fact that
    %x, %y, and %z were already doing so for the human-readable variant.

means considering whether to do the same sort of thing with the
newly-added %W (birth time/crtime).  Note that %W currently expands to "-"
on systems with no support (e.g., linux).

I don't particularly like the idea of making stat do this:

    $ src/stat -c %W.%:W .
    -.-

Rather, I have a slight preference to make it do this:

    $ src/stat -c %W.%:W .
    0.000000000

In any case, I think %w should still expand to "-".

The alternative is to leave %W separate, with no %:W variant.
I.e., %W would continue to print floating point seconds.
In that case, it would be inconsistent with %X, %Y and %Z.

Other preferences?