Remove branches for setting booleans
Swap negative comparison

Check whether this is a symbolic link, not that it isn’t one
Error handling based on return values not error byref
Support relative symlink destinations

I couldn’t see how to make
`createSymlinkAtURL:withDestinationURL:error:` to make a relative
destination symlink, I tried using `URLWithString:relativeToURL:` with
no joy
Merge pull request #2 from github/fix-symlink

Fix symlink support