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>

docs/reference/commandline/dockerd.md

@@ -54,9 +54,10 @@ weight = -1
       --label=[]                             Set key=value labels to the daemon
       --log-driver="json-file"               Default driver for container logs
       --log-opt=[]                           Log driver specific options
-      --mtu=0                                Set the containers network MTU
       --max-concurrent-downloads=3           Set the max concurrent downloads for each pull
       --max-concurrent-uploads=5             Set the max concurrent uploads for each push
+      --mtu=0                                Set the containers network MTU
+      --oom-score-adjust=-500                Set the oom_score_adj for the daemon
       --disable-legacy-registry              Do not contact legacy registries
       -p, --pidfile="/var/run/docker.pid"    Path to use for daemon PID file
       --raw-logs                             Full timestamps without ANSI coloring
@@ -1057,6 +1058,7 @@ This is a full example of the allowed configuration options on Linux:
 	"insecure-registries": [],
 	"disable-legacy-registry": false,
 	"default-runtime": "runc",
+	"oom-score-adjust": -500,
 	"runtimes": {
 		"runc": {
 			"path": "runc"