main: exit 0 on -h/--help

This restores parity with 0.4.x:
```
; bin/etcd-0.4.6  --help &> /dev/null ; echo $?
0
```

... as well as most standard *nix tools:
```
; for cmd in ls ps df wc systemctl ; do $cmd --help &>/dev/null; echo $?; done
0
0
0
0
0
```

I don't know why golang chooses to exit 2 by default [1] even if it's `ErrHelp`; I consider that a bug. (Maybe because they expect most use cases to use a FlagSet and override the error handling?)

[1]: http://golang.org/src/pkg/flag/flag.go?s=26091:26140#L802