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 printed it because the client can use it in a subsequent curl command to:
http://.../exec/{id}/json
to inspect the state (and exit code) of the exec command.  Similar to what people can do for container IDs.