Support setting OOM score

[phemmer]

One thing that would be useful is prioritizing certain containers when it comes to the OOM killer. Right now, docker only supports a simple on/off. It would be much more useful to be able to score containers, so that if a process has to be killed, it favors certain containers.
This behavior is controlled by the /proc/$pid/oom_score_adj tunable. A score of -1000 disables the OOM killer, and the max score of 1000 makes it a near certain target for the OOM killer.

Additionally, since -1000 disables the OOM killer, this setting would deprecate the current --oom-kill-disable parameter.

[thaJeztah]

For reference; earlier discussion on this topic here: #12528.

The --oom-kill-disable flag was introduced in #11034, and isn't in a release yet, so _if_ this is something that's wanted, it would be preferable to decide on this before the 1.7 release, to prevent having to introduce and deprecate that flag.

/cc @HuKeping @hqhq @thockin @LK4D4

[hqhq]

Yes, oom_score_adj is more flexible, technically is't per process, while oom-kill-disable is per cgroup, but we can hack oom_score_adj to make it per cgroup because it will be inherited to child processes.
I'm +1 to add oom_score_adj support, but I don't think it would deprecate oom-kill-disable, because oom-kill-disable is more obvious and clear for what it does. And I think kernel support both of them for some reasons, I don't remember either of them is for backward compatibility.