Add support for docker exec to return cmd exitStatus

Closes #8703

Also made it so that when "docker exec" is run with -d (detached mode)
it'll print the id of the exec to the screen like "docker run -d" does
so people can query it via:
    curl http://.../exec/{id}/json

Signed-off-by: Doug Davis <dug@us.ibm.com>
I don't want it removed when the exec command finishes, I need it to be around afterwards so that people can query the exit code.  Unless we add a "docker exec-rm..." type of API, I think the best place to remove it is when the container is deleted.