cluster: rewrite debug ports consistently

When debug flags are passed to clustered applications, the debug
port is rewritten for each worker process to avoid collisions.
Prior to this commit, each debug flag would get a unique value.
This commit reworks the logic to assign the same port value to
all debug flags for a single worker.

PR-URL: https://github.com/nodejs/node/pull/7050


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>