Fix service restart over salt

When you attempt to restart a salt minion using salt it fails to restart properly.

```
[root@host ~]# salt 'foo' cmd.run 'service salt-minion restart'
db1:
    Stopping salt-minion daemon: [  OK  ]
    Starting salt-minion daemon: already running
```

It thinks it's already running because pidofproc mistakes the process that is running the restart command for the main salt-minion process.

The fix is to help pidofproc identify the main salt-minion process by telling it the pidfile to use so it doesn't have to guess.

Tested on CentOS 6.5