net: Replace ifname check with IFF_LOOPBACK in Discover

Checking the interface name is kind of brittle. In the age of network
namespaces and containers, there is no reason a loopback interface can't
be called differently.

Check for the `IFF_LOOPBACK` flag to detect loopback interface instead.

Author: laanwj

src/net.cpp

@@ -3077,8 +3077,7 @@ void Discover()
         {
             if (ifa->ifa_addr == nullptr) continue;
             if ((ifa->ifa_flags & IFF_UP) == 0) continue;
-            if (strcmp(ifa->ifa_name, "lo") == 0) continue;
-            if (strcmp(ifa->ifa_name, "lo0") == 0) continue;
+            if ((ifa->ifa_flags & IFF_LOOPBACK) != 0) continue;
             if (ifa->ifa_addr->sa_family == AF_INET)
             {
                 struct sockaddr_in* s4 = (struct sockaddr_in*)(ifa->ifa_addr);