Add ability to overwrite regular files and symbolic links in NIO FileSystem

[stepan-ulyanin]

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. Implements atomic overwrite for both regular files and symbolic links.
   1. On Darwin a COPYFILE_UNLINK flag is added to copyfile(2)
   2. On Linux platforms an atomic rename is used: the source is copied to a temporary destination first and then atomically renamed, using rename(2) which overwrites an existing file or symbolic link.
3. Adds test cases to the integration test suite.
4. Updates the _NIOFileSystem FileSystem's copyItem for backward compatibility.

Tests

• testCopyFileOverwritingExistingFile
• testCopyFileOverwritingNonExistingFile
• testCopyFileOverwritingCleansUpTempFile
• testCopySymlinkOverwritingExistingSymlink
• testCopySymlinkOverwritingNonExistingSymlink
• 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.