flynn-host run command

This includes a few fixes for exec and containerinit, as well as a new command, `flynn-host run` that runs interactive jobs on the cluster. It is very useful for debugging low-level issues that prevent bootstrap from succeeding.
Jobs which exit non-zero leave the terminal in raw mode, it's a bit hard to show but try running:

```
flynn-host run $IMAGE bash -c 'exit 1'
```

After that my terminal is in noecho mode and I need to `reset`.