UV_FS_COPYFILE_FICLONE attemps to use copy-on-write
semantics in uv_fs_copyfile(). If CoW is not available,
it falls back to a normal copy operation.
Refs: https://github.com/libuv/libuv/pull/1465
PR-URL: https://github.com/libuv/libuv/pull/1491
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>