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>
Because we don't know for sure that it will only be retrieved once.  For example, the docker cli retrieves it automatically to set its own exitStatus on the "docker exec" command but how do we know that some other process on the client side might not do its own inspection later on.
The amount of data kept around is minimal so I don't really see an issue.  However, and as was mentioned before, if we want we could add a "docker exec-rm" type of command to drop all metadata about the cmd if we really wanted.
But, having said all of that, if the maintainers want it cleaned-up after its been retrieved once I can do that.