refactor: move network monitoring to separate goroutine

- Move network monitoring initialization out of serveDNS() function
- Start network monitoring in a separate goroutine during program startup
- Remove context parameter from monitorNetworkChanges() as it's not used
- Simplify serveDNS() function signature by removing unused context parameter
- Ensure network monitoring starts only once during initial run, not on reload

This change improves separation of concerns by isolating network monitoring
from DNS serving logic, and prevents potential issues with multiple
monitoring goroutines if starting multiple listeners.

cmd/cli/prog.go

@@ -514,6 +514,12 @@ func (p *prog) run(reload bool, reloadCh chan struct{}) {
 	for listenerNum := range p.cfg.Listener {
 		p.cfg.Listener[listenerNum].Init()
 		if !reload {
+			go func() {
+				// Start network monitoring
+				if err := p.monitorNetworkChanges(ctx); err != nil {
+					mainLog.Load().Error().Err(err).Msg("Failed to start network monitoring")
+				}
+			}()
 			go func(listenerNum string) {
 				listenerConfig := p.cfg.Listener[listenerNum]
 				upstreamConfig := p.cfg.Upstream[listenerNum]