Support execution as unprivileged user?

Hi there,

This is exactly what I was looking for - however, I am trying to run (all of) my gameservers in Kubernetes. Depending on the cluster configuration, you may not even be able to run a container as root. Ideally, you would be able to run the containers with a `securityContext` enforcing an unprivileged uid. In situations like these, `gosu` will fail outright.

I think this can be as easy as wrapping `exec gosu` usage in various places with a function call that basically does:

```bash

if [ -z $NO_GOSU ]; then
exec gosu ...
else
raw_command
fi
```

I am happy to submit a PR but wanted to get thoughts on adding this before doing it.

Thanks!