patch: net, net/http: add enforcement hooks

[bradfitz]

Summary: let us add some safety checks inside our control plane that people don't accidentally introduce network operations while holding locks or inside database transactions
Upstream status: seems unlikely to be accepted
Importance: safety check for corp only (https://github.com/tailscale/corp/issues/8944), not tailscale/tailscale.
Example prior commit: 13373ca and #68