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>

daemon/config_unix.go

@@ -34,6 +34,7 @@ type Config struct {
 	Ulimits              map[string]*units.Ulimit `json:"default-ulimits,omitempty"`
 	Runtimes             map[string]types.Runtime `json:"runtimes,omitempty"`
 	DefaultRuntime       string                   `json:"default-runtime,omitempty"`
+	OOMScoreAdjust       int                      `json:"oom-score-adjust,omitempty"`
 }
 
 // bridgeConfig stores all the bridge driver specific
@@ -90,6 +91,7 @@ func (config *Config) InstallFlags(cmd *flag.FlagSet, usageFn func(string) strin
 	config.Runtimes = make(map[string]types.Runtime)
 	cmd.Var(runconfigopts.NewNamedRuntimeOpt("runtimes", &config.Runtimes, stockRuntimeName), []string{"-add-runtime"}, usageFn("Register an additional OCI compatible runtime"))
 	cmd.StringVar(&config.DefaultRuntime, []string{"-default-runtime"}, stockRuntimeName, usageFn("Default OCI runtime to be used"))
+	cmd.IntVar(&config.OOMScoreAdjust, []string{"-oom-score-adjust"}, -500, usageFn("Set the oom_score_adj for the daemon"))
 
 	config.attachExperimentalFlags(cmd, usageFn)
 }