FTP: fix IPv6 host using link-local address

... and make sure we can connect the data connection to a host name that
is longer than 48 bytes.

Also simplifies the code somewhat by re-using the original host name
more, as it is likely still in the DNS cache.

Original-Patch-by: Vojtěch Král
Bug: http://curl.haxx.se/bug/view.cgi?id=1468