When copying the current directory (.) to an existing directory, ensure
that the directory name is properly stripped from the descendant path
and the context is set up correctly. This matches GNU cp behavior.

- Set root_parent to None when copying . to existing directory
- Strip current directory name from descendant path in Entry::new
- Return target path directly in construct_dest_path for . -> dir case
- Fix panic when root.file_name() is None for . path

This commit adds tests to verify that file permissions and timestamps are preserved when copying the current directory (.) to a destination. Specific permissions and timestamps are set on the source files, and assertions are made to ensure that these attributes are correctly maintained in the copied files. The tests are conditionally compiled for non-Windows and non-FreeBSD systems.