Degrade gracefully when PID argument is a kernel thread

We currently do not deal very well with PID arguments that correspond to kernel threads. The desired output is below:

```
$ pargs 2
2: pageout
$ pauxv 3
3: fsflush
$ penv 3
3: fsflush
$ pfiles 3
3:      fsflush
  [system process]
$ pstack 3
pstack: cannot examine 3: system process
```