debugger: don't spawn child process in remote mode

When debug in remote mode with host:port or pid, the interface
spawn child process also. If the debugger agent is running, will
get following output:

```
< Error: listen EADDRINUSE :::5858
<     at Object.exports._errnoException (util.js:734:11)
<     at exports._exceptionWithHostPort (util.js:757:20)
<     at Agent.Server._listen2 (net.js:1155:14)
<     at listen (net.js:1181:10)
<     at Agent.Server.listen (net.js:1268:5)
<     at Object.start (_debug_agent.js:21:9)
<     at startup (node.js:68:9)
<     at node.js:799:3
```

This fix won't spawn child process and no more error message was
shown.

Reviewed-By: Julien Gilli <julien.gilli@joyent.com>
PR-URL: https://github.com/joyent/node/pull/14172