xz: Make --keep accept symlinks, hardlinks, and setuid/setgid/sticky.

xz: Make --keep accept symlinks, hardlinks, and setuid/setgid/sticky.

Previously this required using --force but that has other
effects too which might be undesirable. Changing the behavior
of --keep has a small risk of breaking existing scripts but
since this is a fairly special corner case I expect the
likehood of breakage to be low enough.

I think the new behavior is more logical. The only reason for
the old behavior was to be consistent with gzip and bzip2.

Thanks to Vincent Lefevre and Sebastian Andrzej Siewior.