ipn/ipnserver: make Engine argument a func that tries again for each connection

So a backend in server-an-error state (as used by Windows) can try to
create a new Engine again each time somebody re-connects, relaunching
the GUI app.

(The proper fix is actually fixing Windows issues, but this makes things better
in the short term)

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnserver/server_test.go

@@ -72,6 +72,6 @@ func TestRunMultipleAccepts(t *testing.T) {
 		SocketPath: socketPath,
 	}
 	t.Logf("pre-Run")
-	err = ipnserver.Run(ctx, logTriggerTestf, "dummy_logid", opts, eng)
+	err = ipnserver.Run(ctx, logTriggerTestf, "dummy_logid", ipnserver.FixedEngine(eng), opts)
 	t.Logf("ipnserver.Run = %v", err)
 }