When tailscaled exits unexpectedly (crashes, killed directly), containerboot now exits with a non-zero code to signal failure to the orchestrator. The reaper now distinguishes between graceful shutdowns which still exit 0, and unexpected exits which propagate the child's exit code or force 1 if the child exited cleanly on its own. Fixes#17650
Signed-off-by: Raj Singh <raj@tailscale.com>