lib: rename validateMode to parseMode

The function did not only validate the mode but it returns a new
value depending on the input. Thus `validate` did not seem to be an
appropriate name.

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>