fix handling of dns response codes

rcode of 3 (NxDomain) was treated as a hard EAI_NONAME failure, but it
should instead return 0 (no results) so the caller can continue
searching. this will be important for adding search domain support.
the top-level caller will automatically return EAI_NONAME if there are
zero results at the end.

also, the case where rcode is 0 (success) but there are no results was
not handled. this happens when the domain exists but there are no A or
AAAA records for it. in this case a hard EAI_NONAME should be imposed
to inhibit further search, since the name was defined and just does
not have any address associated with it. previously a misleading hard
failure of EAI_FAIL was reported.

src/network/lookup_name.c

@@ -152,7 +152,8 @@ static int name_from_dns(struct address buf[static MAXADDRS], char canon[static
 
 	if (ctx.cnt) return ctx.cnt;
 	if (alens[0] < 4 || (abuf[0][3] & 15) == 2) return EAI_AGAIN;
-	if ((abuf[0][3] & 15) == 3) return EAI_NONAME;
+	if ((abuf[0][3] & 15) == 0) return EAI_NONAME;
+	if ((abuf[0][3] & 15) == 3) return 0;
 	return EAI_FAIL;
 }