ipn/ipnlocal,wgengine/netstack: move LocalBackend specifc serving logic to LocalBackend

The netstack code had a bunch of logic to figure out if the LocalBackend should handle an
incoming connection and then would call the function directly on LocalBackend. Move that
logic to LocalBackend and refactor the methods to return conn handlers.

Updates #cleanup

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ipn/ipnlocal/peerapi.go

@@ -780,7 +780,7 @@ func (h *peerAPIHandler) handleServeIngress(w http.ResponseWriter, r *http.Reque
 		return
 	}
 
-	getConn := func() (net.Conn, bool) {
+	getConnOrReset := func() (net.Conn, bool) {
 		conn, _, err := w.(http.Hijacker).Hijack()
 		if err != nil {
 			h.logf("ingress: failed hijacking conn")
@@ -798,7 +798,7 @@ func (h *peerAPIHandler) handleServeIngress(w http.ResponseWriter, r *http.Reque
 		http.Error(w, "denied", http.StatusForbidden)
 	}
 
-	h.ps.b.HandleIngressTCPConn(h.peerNode, target, srcAddr, getConn, sendRST)
+	h.ps.b.HandleIngressTCPConn(h.peerNode, target, srcAddr, getConnOrReset, sendRST)
 }
 
 func (h *peerAPIHandler) handleServeInterfaces(w http.ResponseWriter, r *http.Request) {