cp: Fix broken symlinks to parent-dir

[luigieli]

According to the GNU cp official documentation, a symlink is only created if all source paths are absolute, unless the destination files are in the current directory.

This fix solves this GNU cp incompatibility by:

• Adding verifications when handling symlinks, to ensure that a symlink with relative path are only created if the destination is within the current directory.
• Adding unit tests to cover this modifications, and ensure the behavior is correctly align with GNU documentation.

Fixes #6385.

[github-actions[bot]]

GNU testsuite comparison:

GNU test failed: tests/timeout/timeout. tests/timeout/timeout is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)

[github-actions[bot]]

GNU testsuite comparison:

Congrats! The gnu test tests/timeout/timeout is no longer failing!