(Recent) USBNet bundles a full-featured cURL build. It's not symlinked (probably because the Kindle already ships one, I didn't even remember that

), but it's in /mnt/us/usbnet/bin (and should have HTTPS support out of the box, too).
Sidebar: I'm not a fan of FTP

. You could go with SCP/SFTP, or rsync, or, hell, hide it all behind an sshfs mount

.