Time: tztime: mention IANA TZ Database in help

https://www.iana.org/time-zones uses the spelling IANA Time Zone Database,
so I use it too.
Time: update messages.pot & l10n-fi

I translated IANA Time Zone Database too, because I tried googling for
my translation and the first results are about correct thing (even if
Google first suggests wrong spelling which (when clicked) suggests two
other wrong spellings…

[SKIP CI]