Give containers unique machine-id based off container ID

Currently, all containers on a host share a single machine ID, accessible in /etc/machine-id. This patch adds a per-container unique ID (formed by truncating the container's ID). Programs which report information based off or identified by machine-id can now distinguish which container they are in.

Major application for this is logging with journald - these logs are distinguished based on machine-id, and with a unique per-container machine-id and a few symlinks, an in-container systemd instance can send logs (uniquely tied to the generating container) to the host.