Change Shim OOM score to +1 of containerd

[sipsma]

Containerd recently updated their reference shim implementations to set the OOM score of the shim to +1 that of the parent containerd process:

This should allow the shim's to
be killed first in an OOM condition but leave the daemon alone for a bit
to help cleanup and manage the containers during this situation.

We should probably make the same change here but can't quite re-use their code because the fc-control plugin actually runs in the same process as containerd, so it needs to set the shim score to +1 of its own (not of the parent process).

[praveensastry]

@sipsma I'd like to contribute to this.

[sipsma]

@praveensastry Sounds great, thanks! Please feel free to reach out on this issue or email if we can help clarify anything in the process

[sipsma]

Closed by #238