lib: add 'pid' prefix in `internal/util`

This PR improves `prefix` in `util` that we've agreed on
https://github.com/nodejs/node/pull/3833
(separate code for javascript to move the printing function
to C++ directly)

PR-URL: https://github.com/nodejs/node/pull/3878


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>