Provide Out Of Memory information in container status

Container processes can get killed by the kernel whenever they exceed their memory limit or when the overall memory available in the system is very low. When containers get killed by the kernel, there is no way to figure out if it died due to lack of memory.
To solve this problem, this patch modified docker to Identify out of memory kills inside containers and indicate that as part of the container status string. Since some containers can gracefully handle OOM kills, this patch does not attempt to bring down a container whenever an OOM occurs.

Ping @crosbymichael @vieux 
cc @bgrant0607
We are already waiting for the container death. I do agree that there could be a race here between the gothreads and the process exit notification from kernel. I have modified the code to wait for the oom notification channel to be closed before returning. Does this look good?