[client] Check for fwmark support and use fallback routing if not supported (#3220)

util/net/env_generic.go

@@ -0,0 +1,12 @@
+//go:build !linux || android
+
+package net
+
+func Init() {
+	// nothing to do on non-linux
+}
+
+func AdvancedRouting() bool {
+	// non-linux currently doesn't support advanced routing
+	return false
+}