cp: Should refuse to create broken symlinks to parent-dir

Copying files as symbolic links to a new (sub-)directory would require constructing a relative path from the new location to the old, e.g. starting with ../.

Trying to do so would necessarily be very fragile. GNU cp even refuses to attempt it. However, uutils currently does it, very badly, resulting in completely broken links and exit code 0.

$ mkdir newdir  
$ cp -vs README.md newdir/  # GNU refuses to create broken symbolic links
'README.md' -> 'newdir/README.md'
cp: newdir/README.md: can make relative symbolic links only in current directory
[$? = 1]
$ ls newdir/  # It's empty
$ cargo run -q cp -vs README.md newdir/  # uutils returns "success"
'README.md' -> 'newdir/README.md'
$ ls newdir/  # And creates something
README.md
$ readlink newdir/README.md  # It creates a self-link
README.md
$ less newdir/README.md 
newdir/README.md: Too many levels of symbolic links
[$? = 1]

Discovered while reviewing #6380.

Start with the cp command entry point and reproduce the issue using the cargo run -q cp -vs README.md newdir/ example. Trace relative symlink handling for a destination subdirectory, then verify that the command refuses the operation with a nonzero exit status and does not create the broken link.