Set oom score for containerd and shims

This adds a config option to set the oom score for the containerd daemon
as well as automatically setting the oom score for the shim's lauched so
that they are not killed until the very end of an out of memory
condition.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

linux/shim.go

@@ -51,6 +51,9 @@ func newShim(path string, remote bool) (shim.ShimClient, error) {
 	if err := reaper.Default.Start(cmd); err != nil {
 		return nil, errors.Wrapf(err, "failed to start shim")
 	}
+	if err := sys.SetOOMScore(cmd.Process.Pid, sys.OOMScoreMaxKillable); err != nil {
+		return nil, err
+	}
 	return connectShim(socket)
 }