Implement copy_file_range()

[ArniDagur]

This should fix the problems with #971 and #1008.

[asomers]

You'll need to rebase and fix the CHANGELOG entry. Also, please wrap everything to 80 columns.

[asomers]

When I said "wrap everything to 80 columns" I meant to only wrap new code, not existing code. Now you have a big change that mixes style with content. That's not good. Please undo it. If you want to wrap the rest of the file, that should be done as a separate commit.

[ArniDagur]

This should be fine, right?

[asomers]

It all LGTM now, but you need to rebase to fix the CHANGELOG.

[ArniDagur]

I have rebased and fixed the changelog.

[asomers]

bors r+

[bors]

Build succeeded

• continuous-integration/travis-ci/push
• FreeBSD 11.2

[ArniDagur]

@asomers It seems that I made a mistake in this PR: The function should be in unistd.h, not fcntl.h. Perhaps this should be fixed in the next major version?

[asomers]

Honestly I wouldn't bother. The user disruption that would come by renaming the symbol isn't worth the gain to consistency.

[dubiousjim]

Why not export it in the correct place, and re-export it in nix::fcntl with a comment in the docs to say this export will eventually go away.