Stop the inbound accept loop from spamming errors on shutdown

[Bornoz]

On shutdown the inbound listener is closed and listener.Accept() returns use of closed network connection. The accept loop logged that as ERROR, waited 5s, and retried forever, so a clean shutdown produced a wall of identical ERROR lines (exactly the output in #230).

The package already defines ErrListenerClosed for this string. When Accept fails with a closed listener, log a single debug line and return to end the loop; every other error keeps the existing back-off-and-retry path.

go build ./p2p/ and gofmt are clean, and go vet ./p2p/ reports nothing new on the changed file (the two pre-existing set_test.go warnings are unrelated).

Closes #230

[ygd58]

Looks like this overlaps with #521, which I opened earlier for the same root cause (#230) — flagging for whoever reviews these, not asking for either to be closed.

One technical difference worth noting: ErrListenerClosed here is a string constant matched via strings.Contains(err.Error(), ...), which predates net.ErrClosed (added in Go 1.16 specifically to replace string-matching this exact condition). errors.Is(err, net.ErrClosed) correctly traverses wrapped errors regardless of message text, so it's a bit more robust against future error-wrapping changes upstream (in netutil.LimitListener or Go's net package itself) than matching on message text — though both work correctly against the current Accept() behavior.

#521 also covers the other half of #491/#230: cmd/rpc.Server.updatePollResults panicking on a store-closed-during-shutdown race (via store.NewSMT()), plus test coverage for the accept-loop exit behavior specifically (asserting the goroutine actually returns instead of hanging, using a real net.Listener).