src: make debugger listen on 127.0.0.1 by default

Commit 22720524 ("net: bind to `::` TCP address by default") from
April 2014 seems to have accidentally changed the default listen
address from 127.0.0.1 to 0.0.0.0, a.k.a. the "any" address.

From a security viewpoint it's undesirable to accept debug agent
connections from anywhere so let's change that back.  Users can
override the default with the `--debug=<host>:<port>` switch.

Fixes: https://github.com/nodejs/node/issues/8081
PR-URL: https://github.com/nodejs/node/pull/8106


Reviewed-By: James M Snell <jasnell@gmail.com>