initial servicebuilder integration

A simple library for writing servicebuilder configurations

```go
import "github.com/platypus-platform/pp/pkg/runit"

builder := runit.DefaultBuilder
template := runit.NewSBTemplate("exemplar")
template.AddEntry("exemplar", []string{"/data/app/exemplar/bin/launch"})
err := builder.Write(template)
```
should doc that this (or maybe what uses it) isn't atomic but that's justified because we're writing to a staging directory.