Refactor: TPROXY inbound UDP write back

https://t.me/projectXray/119670

虽然不一定是最终的版本，但值得记录，感谢协助测试的各位朋友，特别感谢 @yichya @huyz

proxy/dokodemo/fakeudp_other.go

@@ -0,0 +1,12 @@
+// +build !linux
+
+package dokodemo
+
+import (
+	"fmt"
+	"net"
+)
+
+func FakeUDP(addr *net.UDPAddr, mark int) (net.PacketConn, error) {
+	return nil, &net.OpError{Op: "fake", Err: fmt.Errorf("!linux")}
+}