Fix keyring issue and revert to systemd

- Reverted to systemd init system (more compatible)
- Updated package list: live-config-systemd, systemd-sysv
- Removed problematic devuan-keyring hook that was interfering with bootstrap
- Kept debian-keyring and debian-archive-keyring for proper Debian verification
- Updated README to reflect systemd usage
- This should resolve the keyring signature verification error