Adds a new parameter to the NIOFS FileSystem's copyItem that allows to atomically overwrite regular files and symbolic links.

Note that I opted in to not implement the overwrite for directories in this PR as it seems that there is no atomic way to overwrite directory trees on Posix systems - do we want to do it non-atomically?

Motivation:

Per #3403 it is not uncommon to want to copy an item and not care if the copied item overwrites an existing one.

Modifications:

1. Adds a new overwriting: Bool parameter to NIOFS FileSystem's copyItem.
2. Adds new syscall wrappers for TOCTOU-protected atomic operations:
   1. system_renameatx_np / Syscall.rename(from:relativeTo:to:relativeTo:options:) - Darwin directory-relative rename
   2. system_symlinkat / Syscall.symlinkat - create symlink relative to directory FD
   3. system_unlinkat / Syscall.unlinkat - unlink file relative to directory FD
3. Implements atomic overwrite for both regular files and symbolic links.
   1. For regular files:
      1. On Darwin: a COPYFILE_UNLINK flag is added to copyfile(2)
      2. On Linux: an atomic rename is used - the file is copied to a temporary destination first and then atomically renamed, using renameat2(2) which overwrites the existing file.
   2. For symlinks:
      1. On Darwin: an atomic rename is used - the symlink is copied to a temporary destination first and then atomically renamed, using renameatx_np(2) which overwrites the existing symlink.
      2. On Linux: an atomic rename is used - the symlink is copied to a temporary destination first and then atomically renamed, using renameat2(2) which overwrites the existing symlink.
4. Adds test cases to the integration test suite.
5. Updates the _NIOFileSystem FileSystem's copyItem for backward compatibility.

Tests

Syscall tests:

• test_symlinkat
• test_unlinkat
• test_renameatx_np

Integration tests:

• testCopyFileOverwritingExistingFile
• testCopyFileOverwritingNonExistingFile
• testCopyFileOverwritingCleansUpTempFile
• testCopyFileOverwritingPreservesPermissions
• testCopySymlinkOverwritingExistingSymlink
• testCopySymlinkOverwritingNonExistingSymlink
• testCopySymlinkOverwritingCleansUpTempLink
• testCopyFileOverwritingExistingSymlink
• testCopySymlinkOverwritingExistingFile

Result:

Users will be able to copy a regular file or a symbolic link, overwriting an existing regular file or existing symbolic link.