util: fix inspecting of proxy objects

In certain conditions, inspecting a Proxy object can lead to a
max call stack error. Avoid that by detecting the Proxy object
and outputting information about the Proxy object itself.

Fixes: https://github.com/nodejs/node/issues/6464
PR-URL: https://github.com/nodejs/node/pull/6465


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>