[PATCH 7 of 7 V3] util: enable hardlink for copyfile

[Jun Wu]

Excerpts from Augie Fackler's message of 2017-03-12 14:03:44 -0400:
> It's actually believed the problem is in the Windows CIFS *server*, if
> you read the bug attached to the previous change.
> 
> I'm hesitant to turn on hardlinks at all because of the way problems
> manifest (unrecoverable data corruption for users, at some random
> point in the future), so if we're going to do it we need to be very
> conservative to avoid the bug again.

The code only affects transaction backing up for atomatictemp files. I'm not
sure how is it related to manifests.

It is very conservative. It makes sure the information is correct, unless a
power user wants to break us intentionally. None of the network filesystems
(fuse-sshfs, cifs, nfs) is considered safe. And non-Linux systems remain
unsafe. The comment about CIFS may be inaccurate, I can update it. But Patch
1 should be very solid.

Modern filesystems will have new features outside POSIX. For example, if
btrfs becomes more popular, we may have fancy transaction support.