src,lib: minor --debug-brk cleanup

Minor cleanup of how --debug-brk works:
* We no longer need to use command line flags to expose the debug
  object.
* Do not depend on the existence of global.v8debug as a mechanism to
  determine if --debug-brk was specified.
* We no longer need to set a dummy listener with --debug-brk.

PR-URL: https://github.com/nodejs/node/pull/6599


Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: cjihrig - Colin Ihrig <cjihrig@gmail.com>