Like the upcoming version of GNU coreutils 'cp' prefer a copy-on-write clone.
This shares the physical storage between files, which means no data needs to copied.
CoW-clones are supported by a number of Linux file systems, like Btrfs, XFS, and overlayfs.

Eliminate a 'stat' call that is always performed for checking if the target is a directory
by only performing the check when the 'open' syscall reports an error.

Eliminate a 'stat' call for retrieving the file size of the source by passing through
the length that was retrieved when checking the opened file is not a directory.

Create the destination with file permissions that match the source.
We still need to fchmod due to umask being applied to the open mode.

When performing a manual copy, limit the allocated buffer for small files.
And, avoid the last 'read' call by checking when we've copied the expected nr of bytes.