Changed min duration to 1 & added code to accept an int in addition to duration

This could be confusing if someone expects the value to be an int but accidentally adds extra characters. For example:

```
remote_syslog -D --new-file-check-interval=1q
invalid argument "1q" for --new-file-check-interval=1q: time: unknown unit q in duration 1q
```

Might be worth fully switching over to int. @eric what do you think?
I would move the duration check and set to a `SetDuration(time.Duration)` function.