lib: make --debug-port work with cluster

Make the cluster module intercept the `--debug-port=<port>` command line
switch and replace it with the debug port of the child process.

A happy coincidence of this change is that it finally makes it possible
to run the sequential/test-debug-signal-cluster in parallel, it now no
longer needs the default port numbers.

PR-URL: https://github.com/iojs/io.js/pull/306


Reviewed-By: Miroslav Bajtoš <miroslav@strongloop.com>