Reads the contents of the symbolic link referred to by path. The callback gets two arguments (err, linkString).
See the POSIX readlink(2) documentation for more details.
The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path passed to the callback. If the encoding is set to 'buffer', the link path returned will be passed as a Buffer object.