cluster: do not unconditionally set --debug-port

Currently, each cluster worker is assigned an ever increasing
--debug-port argument. A long running cluster application that
does not use the debugger can run into errors related to the
port range. This commit mitigates the problem by only setting
the debug port if the master is started with debug arguments, or
the user explicitly defines debug arguments for the worker. This
commit also adds a new debug port offset counter that is only
incremented when a worker is created that utilizes debugging.

Fixes: https://github.com/joyent/node/issues/8159
Refs: https://github.com/nodejs/io.js/pull/1524
PR-URL: https://github.com/nodejs/io.js/pull/1949


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Oleg Elifantiev <oleg@elifantiev.ru>