url: change hostname regex to negate invalid chars

Regarding joyent/node#8520

This approach changes hostname validation from a whitelist approach
to a blacklist approach as described in https://url.spec.whatwg.org/#host-parsing.

url.parse misinterpreted `https://good.com+.evil.org/`
as `https://good.com/+.evil.org/`.  If we use url.parse to check the
validity of the hostname, the test passes, but in the browser the
user is redirected to the evil.org website.