Fix bug to allow for multiple PIDs to be returned

Bug:
```
[root@pronghorn04 ~]# service salt-minion restart
Stopping salt-minion daemon:                               [  OK  ]
Starting salt-minion daemon: /etc/init.d/salt-minion: line 67: [: 22610:
unary operator expected
                                                           [  OK  ]
```

Traced bug:
```
++ pidof -c -o 21702 -o 21341 -o %PPID -x salt-minion
+ '[' 22610 7606 ']'
/etc/init.d/salt-minion: line 67: [: 22610: unary operator expected
```