url: refactor validateHostname

This function did not only validate the input but it returned a new
value in case the hostname was valid. This simplifies the function
by always returning the required value, no matter if it is valid or
invalid, so the callee site does not have to check that anymore. On
top the function is renamed to `getHostname` to make clear that the
function does not only validate the input but it also returns a new
value.

PR-URL: https://github.com/nodejs/node/pull/26809


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>