Switch to SysVinit and enhance keyring support

- Changed init system from systemd to sysvinit
- Updated package list: live-config-sysvinit, sysvinit
- Added debian-keyring and debian-archive-keyring for full keyring support
- Created conditional hook for optional devuan-keyring installation
- Updated README with sysvinit and keyring information
- Maintained all existing functionality with improved compatibility