net: Lookup does not support Android/ios


<pre>
$ go version
1.12.9
</pre>

```
package main
 
import (
  "fmt"
  "net"
)
 
func main() {
  txtrecords, _ := net.LookupTXT("facebook.com")
 
  for _, txt := range txtrecords {
    fmt.Println(txt)
  }
}
```

`lookup facebook.com on [::1]:53: read udp [::1]:57440->[::1]:53: read: connection refused`

android There is no `/etc/resolv.conf` file.

