Allow the user to set DOCKER_NOWARN_KERNEL_VERSION=1 to disable the
warning for RHEL 6.5 and other distributions that don't exhibit the
panics described in https://github.com/dotcloud/docker/issues/407.
* 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
* Implement a new package: engine. It exposes a useful but minimalist job API.
* Refactor main() to instanciate an Engine instead of a Server directly.
* Refactor server.go to register an engine job.
This is the smallest possible refactor which can include the new Engine design
into master. More gradual refactoring will follow.
