Implement linux copy algorithms using syscalls

[liss-h]

This PR implements the various linux file copy algorithms using syscalls instead of relying on cp.

Implemented algorithms:

• file clone using ioctl(2)
• sparse copy
• in-kernel dense copy using copy_file_range

Additionally, adds a fallback mechanism to the sparse copy and clone function that tries the "worse" options after failing to use the "good" algorithm. E.g. clone tries clone -> sparse_copy -> in-kernel dense copy -> dense-copy. And fails only if all of them fail.

Edit: this is related to #297

[KIwabuchi]

Thanks for the PRs! I was swamped with other things last week. I'll finish the reviews (including the other one) this week. Bear with me...

[KIwabuchi]

This PR looks great to me! I like the new sparse copy functions and the fall-back policy.

I left just a few minor comments.
In addition to that, could you apply clang-format on your side?
There is a clang-format file in the root directory.

[liss-h]

Thanks for reviewing

Regarding running clang-format: the style in the .clang-format file does not seem to match the codebase completely. For instance in .clang-format reference and pointer alignment is configured to left but in the codebase it is actually right so running it produces a lot of changes that are unrelated to this PR.

[KIwabuchi]

That's a catch! I investigated the .clang-format file and found that setting DerivePointerAlignment to true ignores the PointerAlignment option. As it's confusing, I'll update the entire code so that it matches with the 'left' style.

[KIwabuchi]

I'll merge this PR once the CI has passed.