[tz] [PROPOSED] Improve leapseconds support

[Paul Eggert]

Arthur David Olson wrote:
> Perhaps accept leap seconds at the ends of months other than June or
> December, but issue a warning? (And similarly for negative leap seconds?)

Although we could, there's lots of other stuff the script could check for, such 
as dates like "6 Oct 2018" (or "1 Winterfilth, SR 1453"...). Having the script 
properly validate its input (much less warn about implausible input) would 
significantly complicate it. If someone were to write such a validation I 
suppose we would accept it, but given the script's intended use I would give the 
project low priority.

A better line of attack (if someone wants to volunteer) would be for the script 
to parse the input data instead of scraping dates from comments in the input. 
Parsing the comments, as we do now, is surely asking for trouble. For example, 
in this input line:

3692217600	37	# 1 Jan 2017

the script currently ignores the 3692217600 which is the actual data, and parses 
the "1 Jan 2017". Eeeuuw.

Chris Woodbury recently sent me a patch to parse the data, but unfortunately his 
patch relied heavily on nonportable GNU extensions and I'd rather stick with 
plain POSIX tools. Chris also suggested that we stop using the NIST file 
entirely and switch to the USNO format; however that would rattle many of our 
downstream users, and it would possibly make the stick-with-POSIX approach even 
harder.