Mark places in the code for IPv6 support

This commit adds some TODO comments that indicate places in the code that will need to be updated for true IPv6 support. This includes finding the system's nameservers and sending a message over UDP.

It was brought up in GH-25 and I felt like a comment was necessary.

dns-transport/src/udp.rs

@@ -47,6 +47,7 @@ impl UdpTransport {
 impl Transport for UdpTransport {
     fn send(&self, request: &Request) -> Result<Response, Error> {
         info!("Opening UDP socket");
+        // TODO: This will need to be changed for IPv6 support.
         let socket = UdpSocket::bind((Ipv4Addr::UNSPECIFIED, 0))?;
 
         if self.addr.contains(':') {

src/resolve.rs

@@ -51,6 +51,7 @@ fn system_nameservers() -> io::Result<Option<Nameserver>> {
 
         if let Some(nameserver_str) = line.strip_prefix("nameserver ") {
             let ip: Result<std::net::Ipv4Addr, _> = nameserver_str.parse();
+            // TODO: This will need to be changed for IPv6 support.
 
             match ip {
                 Ok(_ip) => nameservers.push(nameserver_str.into()),
@@ -78,6 +79,7 @@ fn system_nameservers() -> io::Result<Option<Nameserver>> {
     for dns_server in adapters
         .iter()
         .flat_map(|adapter| adapter.dns_servers().iter()) {
+            // TODO: This will need to be changed for IPv6 support.
             if dns_server.is_ipv4() {
                 debug!("Found first nameserver {:?}", dns_server);
                 return Ok(Some(dns_server.to_string()))