Add init process for zombie fighting

This adds a small C binary for fighting zombies.  It is mounted under
`/dev/init` and is prepended to the args specified by the user.  You
enable it via a daemon flag, `dockerd --init`, as it is disable by
default for backwards compat.

You can also override the daemon option or specify this on a per
container basis with `docker run --init=true|false`.

You can test this by running a process like this as the pid 1 in a
container and see the extra zombie that appears in the container as it
is running.

```c

int main(int argc, char ** argv) {
	pid_t pid = fork();
	if (pid == 0) {
		pid = fork();
		if (pid == 0) {
			exit(0);
		}
		sleep(3);
		exit(0);
	}
	printf("got pid %d and exited\n", pid);
	sleep(20);
}
```

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

docs/reference/commandline/dockerd.md

@@ -48,6 +48,7 @@ Options:
       -H, --host=[]                          Daemon socket(s) to connect to
       --help                                 Print usage
       --icc=true                             Enable inter-container communication
+      --init                                 Run an init inside containers to forward signals and reap processes
       --insecure-registry=[]                 Enable insecure registry communication
       --ip=0.0.0.0                           Default IP when binding container ports
       --ip-forward=true                      Enable net.ipv4.ip_forward
@@ -1140,6 +1141,7 @@ This is a full example of the allowed configuration options on Linux:
 	"group": "",
 	"cgroup-parent": "",
 	"default-ulimits": {},
+	"init": false,
 	"ipv6": false,
 	"iptables": false,
 	"ip-forward": false,