Allow -t 0 to sleep forever

```
With most distributions, we know better than "after Ns" when the boot
sequence has finished.

This patch from my fixes branch:

http://bazaar.launchpad.net/~scott/sreadahead/fixes/revision/36

allows -t 0 to mean "wait forever" (ie. until SIGTERM)
```

Original issue reported on code.google.com by `key...@gmail.com` on 20 Jul 2009 at 3:49
