Un-document & further deprecate child_process customFds option

The `customFds` option has been marked as deprecated in the docs since v0.5.11.

This PR removes it from tests and docs and adds a `util.deprecate` wrapper to catch its usage.

Note that there are a number of 3rd party tools within node that still make use of it, which I would expect to see updated upstream and re-imported once people start complaining to the respective maintainers.