util: prevent leaking internal properties

This prevents leaking of the internal `inspect()` properties when
using a custom inspect function.

It also aligns the indentation to the way it was in v8.0.0 since
that changed unintentionally. All strings returned by the custom
inspect function will now be indented appropriately to the current
depth.

PR-URL: https://github.com/nodejs/node/pull/24971
Refs: https://github.com/nodejs/node/issues/24765


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>