[tz] [PROPOSED] Improve leapseconds support

[Paul Eggert]

Brian Inglis wrote:
> NTP times can be easily parsed, converted, and formatted using the NTP epoch
> with date -d "1900-01-01 00:00:00+000 + $NTPtime seconds"

Yes, and Chris's script did something along those lines as well. However, that 
usage of the 'date' command relies on a GNU extension, and the script also uses 
the sha1sum command, another GNU extension. Although I develop on GNU/Linux and 
so these usages are not a problem for me, I'd rather the script be kept portable 
to any POSIX platform as much as possible.