util: display Symbol keys in inspect by default

I use symbol key properties. And I find it awful that they do
not show up in inspection. I can alter
`util.inspect.defaultOptions.showHidden` each time I debug. Does
that sound like fun to you? Isn't fun a core principle life?

The way I see it, it is not about the spec or about what is
enumerable/hidden, etc. When inspecting, it is about ease of
access to the information. That's how I see it. Does anyone have
any other thoughts?

Fixes: https://github.com/nodejs/node/issues/9709
PR-URL: https://github.com/nodejs/node/pull/9726


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>