debugger: export `debug_port` to `process`

`process.debug_port` is useful for changing debugger port in runtime,
before starting it (via SIGUSR1).

Using `--port=` argument for debugger repl, tests will run debugger
server on a `common.PORT` (as it usually does for any other servers).

`process._debugEnd()` stops debugger and its server.

* debugger: implemented process._debugEnd(), `node debug --port=5858 app.js`
* test: start debugger repl on common.PORT
* fixes #2613
* fixes #2614