Add configure option to force retry on DNS failure

This is apparently needed on system which keep nameservers specified
in /etc/resolv.conf even when there is no network connection. Should be
used with care as invalid names will be retried forever.

nameserv.c

@@ -56,7 +56,11 @@ DNS_Name2IPAddress(const char *name, IPAddr *addr)
   result = getaddrinfo(name, NULL, &hints, &res);
 
   if (result) {
+#ifdef FORCE_DNSRETRY
+    return DNS_TryAgain;
+#else
     return result == EAI_AGAIN ? DNS_TryAgain : DNS_Failure;
+#endif
   }
 
   for (ai = res; !result && ai != NULL; ai = ai->ai_next) {
@@ -94,8 +98,13 @@ DNS_Name2IPAddress(const char *name, IPAddr *addr)
     return DNS_Success;
   }
 
+#ifdef FORCE_DNSRETRY
+  return DNS_TryAgain;
+#else
   return DNS_Failure;
 #endif
+
+#endif
 }
 
 /* ================================================== */