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.
When use `iojs debug`, the tip information just like this:
```
Usage: iojs debug script.js
```
This fix will display the advance usage also:
```
Usage: iojs debug script.js
iojs debug <host>:<port>
iojs debug -p <pid>
```
Fixes: https://github.com/iojs/io.js/issues/889
PR-URL: https://github.com/iojs/io.js/pull/1282
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
a2ea16838f