common.sh: add is_docker_enabled() and is_vm_manager_enabled() helpers
reading /boot/config/docker.cfg and /boot/config/domain.cfg.
docker_watchdog: exit cleanly if Docker not enabled in Unraid settings.
stability_watchdog: skip Docker daemon check and Docker container stop
if Docker not enabled; skip virsh VM shutdown if VM Manager not enabled.
server_reboot: skip VM shutdown and libvirt stop if VM Manager not enabled.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
