bug#18316: [PATCH] warn on too large file copies

[Paul Eggert]

Adam wrote:
> I know it's a rare scenario

The next time you copy a file containing 19 exabytes let us know.  :-)

If we're going to fix this, I suggest removing the limit entirely; 
that's better than generating a warning if the limit is gone past.  I 
expect there are plenty of places in the code that stop working after 
2**63 bytes, and I'd focus on them first.