Improve generic ipv4+ipv6 support in libbb.

Convert telnet to it. Now this works:
telnetd -b [::1]:1234 - bind to IPv6 non-standard port
telnet [::1]:1234 - connect to IPv6 non-standard port
telnet ::1 1234 - same
This does not require ANY ipv6-specific code in applets
(no struct sockaddr_in6. In fact, no sockaddr_in, too).

networking/telnetd.c

@@ -414,7 +414,7 @@ telnetd_main(int argc, char **argv)
 	if (IS_INETD) {
 		sessions = make_new_session(0, 1);
 	} else {
-		master_fd = create_and_bind_socket_ip4or6(opt_bindaddr, portnbr);
+		master_fd = create_and_bind_stream_or_die(opt_bindaddr, portnbr);
 		xlisten(master_fd, 1);
 		if (!(opt & OPT_FOREGROUND))
 			xdaemon(0, 0);