cmd/tailscale: make conn breaker use tsdial.Dialer's activeSysConns

In 92fc9a01fa2930e86175f2989ebdd83a9d5d94b4 I added a hacky/clever mechanism to break TCP connections in tailscaled by checking the the first 1,000 FDs on Linux & Darwin to ask the kernel if they were TCP and then to just close their FDs directly.

My reviewer forgot to mention that he'd added explicit accounting of all those system connections ~1.5y prior in https://github.com/tailscale/tailscale/commit/5a1ef1bbb9b67f11d625c543f16005c8c4ac41ba in `tsdial.Dialer.activeSysConns`. 😂

That's much less hacky. Switch to that instead.

/cc @maisem