init: replace ctr with a custom client using the containerd client library

Currently it supports only `service start <SERVICE>`, but it could grow e.g.
`stop`, `exec` etc in the future (although you can still use `ctr` for those).

In order to be able to use go-compile.sh the containerd build needs to move
from /root/go to /go as the GOPATH.

The vendoring situation is not ideal, but since this tool wants to be an exact
match for the containerd it seems tollerable to reuse its vendoring.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

pkg/init/etc/init.d/010-containerd

@@ -34,12 +34,9 @@ if [ -d /containers/services ]
 then
 	for f in $(find /containers/services -mindepth 1 -maxdepth 1 | sort)
 	do
-		base="$(basename $f)"
 		/bin/mount --bind "$f/rootfs" "$f/rootfs"
 		mount -o remount,rw "$f/rootfs"
-		log="/var/log/$base.log"
-		ctr run --runtime-config "$f/config.json" --rootfs "$f/rootfs" --id "$(basename $f)" </dev/null 2>$log >$log &
-		printf " - $base\n"
+		service start "$(basename $f)"
 	done
 fi