cmd/geth: enable DNS discovery by default (#20660)

* node: expose config in service context

* eth: integrate p2p/dnsdisc

* cmd/geth: add some DNS flags

* eth: remove DNS URLs

* cmd/utils: configure DNS names for testnets

* params: update DNS URLs

* cmd/geth: configure mainnet DNS

* cmd/utils: rename DNS flag and fix flag processing

* cmd/utils: remove debug print

* node: fix test

node/node.go

@@ -194,7 +194,7 @@ func (n *Node) Start() error {
 	for _, constructor := range n.serviceFuncs {
 		// Create a new context for the particular service
 		ctx := &ServiceContext{
-			config:         n.config,
+			Config:         *n.config,
 			services:       make(map[reflect.Type]Service),
 			EventMux:       n.eventmux,
 			AccountManager: n.accman,