Fix docker init script

This is the PR related to https://github.com/boot2docker/boot2docker/issues/487
In fact, i tried some ps <ID> manually on my command line in b2d 1.2.0, and the exit code was always 0.
I checked the ps help and it seems that the embeded ps doesn't check for process and exit if not existing :
```
boot2docker: 1.2.0
             master : e75396e - Fri Aug 22 06:03:48 UTC 2014
docker@boot2docker:~$ ps --help
BusyBox v1.22.1 (2014-04-04 15:16:09 UTC) multi-call binary.

Usage: ps [-o COL1,COL2=HEADER]

Show list of processes

        -o COL1,COL2=HEADER     Select columns for display

docker@boot2docker:~$
```

That's why i had to male it a bit shell-complicated :-S