appc: detect cycles in CNAME chain

Fixes tailscale/corp#48296
Updates tailscale/corp#48187

Signed-off-by: Michael Ben-Ami <mzb@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f