src: rework early debug signal handling

Instead of installing an early debug signal handler, simply block the
SIGUSR1 signal at start-up and unblock it when the debugger is ready.

Both approaches are functionally equivalent but blocking the signal
accomplishes it in fewer lines of code.

PR-URL: https://github.com/iojs/io.js/pull/615


Reviewed-By: Sam Roberts <sam@strongloop.com>