Engine: integer job status, improved stream API

* Jobs return an integer status instead of a string
* Status convention mimics unix process execution: 0=success, 1=generic error, 127="no such command"
* Stdout and Stderr support multiple thread-safe data receivers and ring buffer filtering

engine/streams.go

@@ -190,4 +190,3 @@ func (o *Output) AddEnv() (dst *Env, err error) {
 	}()
 	return dst, nil
 }
-