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: new command 'stop' gracefully stops a container.
move t from arg to env
Merge pull request #3175 from vieux/engine-job-stop

Move stop to job