fs: clarify fs.link and fs.linkSync arguments

Updates the argument names `srcpath` and `dstpath` to match the more
descriptive `existingPath` and `newPath` in the documentation.

PR-URL: https://github.com/nodejs/node/pull/9145


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>