Add --oom-score-adjust to daemon

This adds an `--oom-score-adjust` flag to the daemon so that the value
provided can be set for the docker daemon's process.  The default value
for the flag is -500.  This will allow the docker daemon to have a
less chance of being killed before containers do.  The default value for
processes is 0 with a min/max of -1000/1000.

-500 is a good middle ground because it is less than the default for
most processes and still not -1000 which basically means never kill this
process in an OOM condition on the host machine.  The only processes on
my machine that have a score less than -500 are dbus at -900 and sshd
and xfce( my window manager ) at -1000.  I don't think docker should be
set lower, by default, than dbus or sshd so that is why I chose -500.

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

cmd/dockerd/daemon_unix.go

@@ -61,6 +61,7 @@ func (cli *DaemonCli) setupConfigReloadTrap() {
 func (cli *DaemonCli) getPlatformRemoteOptions() []libcontainerd.RemoteOption {
 	opts := []libcontainerd.RemoteOption{
 		libcontainerd.WithDebugLog(cli.Config.Debug),
+		libcontainerd.WithOOMScore(cli.Config.OOMScoreAdjust),
 	}
 	if cli.Config.ContainerdAddr != "" {
 		opts = append(opts, libcontainerd.WithRemoteAddr(cli.Config.ContainerdAddr))