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>

hack/make/.binary-setup

@@ -7,3 +7,4 @@ DOCKER_CONTAINERD_BINARY_NAME='docker-containerd'
 DOCKER_CONTAINERD_CTR_BINARY_NAME='docker-containerd-ctr'
 DOCKER_CONTAINERD_SHIM_BINARY_NAME='docker-containerd-shim'
 DOCKER_PROXY_BINARY_NAME='docker-proxy'
+DOCKER_INIT_BINARY_NAME='docker-init'