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: 13373caf926b991ba7d7babe8b5bc79a325c3fad and https://github.com/tailscale/go/pull/68