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>
Upstream-commit: ee3ac3aa66bfb27b7c21dfb253fdaa113baedd4e
Component: engine

components/engine/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'

components/engine/hack/make/binary-daemon

@@ -12,5 +12,5 @@ set -e
 	export BINARY_SHORT_NAME="$DOCKER_PROXY_BINARY_NAME"
 	export SOURCE_PATH='./vendor/src/github.com/docker/libnetwork/cmd/proxy'
 	source "${MAKEDIR}/.binary"
-	copy_containerd "$DEST" 'hash'
+	copy_binaries "$DEST" 'hash'
 )

components/engine/hack/make/install-binary-daemon

@@ -12,4 +12,5 @@ rm -rf "$DEST"
 	install_binary "${DEST}/${DOCKER_CONTAINERD_CTR_BINARY_NAME}"
 	install_binary "${DEST}/${DOCKER_CONTAINERD_SHIM_BINARY_NAME}"
 	install_binary "${DEST}/${DOCKER_PROXY_BINARY_NAME}"
+	install_binary "${DEST}/${DOCKER_INIT_BINARY_NAME}"
 )

components/engine/hack/make/tgz

@@ -53,8 +53,8 @@ for d in "$CROSS/"*/*; do
 		cp -L "$d/$PROXY_BINARY_FULLNAME" "$TAR_PATH/${DOCKER_PROXY_BINARY_NAME}${BINARY_EXTENSION}"
 	fi
 
-	# copy over all the containerd binaries
-	copy_containerd $TAR_PATH
+	# copy over all the extra binaries
+	copy_binaries $TAR_PATH
 
 	if [ "$IS_TAR" == "true" ]; then
 		echo "Creating tgz from $BUILD_PATH and naming it $TGZ"