url: fix surrogate handling in encodeAuth()

Also factor out common parts in querystring and url.

PR-URL: https://github.com/nodejs/node/pull/11161


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>