inspector: listen on process.debugPort

This commit consolidates the debugging port used by the
inspector and Node's legacy debugger.

Fixes: https://github.com/nodejs/node/issues/8201
PR-URL: https://github.com/nodejs/node/pull/8386


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>