refactor(nsupdate): send all commands at once

This ensures `nsupdate` is only called once per IP update (even for both
IPv4 and IPv6 in a single call).

src/nsupdate.rs

@@ -1,5 +1,6 @@
 use std::{
     ffi::OsStr,
+    io::Write,
     net::IpAddr,
     path::Path,
     process::{ExitStatus, Stdio},