(CURLOPT_FTPPORT) didn't work for ipv6-enabed curls if the IP wasn't a
"native" IP while it works fine for ipv6-disabled builds!

In the process of fixing this, I removed the support for LPRT since I can't
think of many reasons to keep doing it and asking on the mailing list didn't
reveal anyone else that could either. The code that sends EPRT and PORT is
now also a lot simpler than before (IMHO).