[tz] Secure FTP support for IANA downloads

[Paul Eggert]

pbassey--- via tz wrote:
> Does your site offer Secure FTP for your IANA downloads and if not, what would it take to enable this protocol so our company could securely download your files?  Currently, we use this URL to download the IANA time zone files which is not secure:ftp://ftp.iana.org/tz/

As far as I know iana.org does not offer SFTP access to tzdb, I suppose due to 
the relative complexity and configuration hassle associated with that protocol. 
Instead, you can access tzdb via HTTPS. This is recommended nowadays and should 
be just as secure as SFTP for downloads. You can also verify each release via a 
GPG signature for added security; this is especially recommended if you still 
must use plain FTP or HTTP for some reason. For details, please see:

https://mm.icann.org/pipermail/tz-announce/2018-May/000050.html