[tz] Using %z in FORMAT column of 'europe' etc.

[Paul Eggert]

[This is a repost with minor updates of 
<https://mm.icann.org/pipermail/tz/2024-March/058790.html>. This repost 
is in a separate email thread, with a clearer "Subject:" line.]

The recent glitches with %z-related lines in the Portugal data lead me
to think that it's time to start using %z in the main format. For
example, instead of this:

> # Zone NAME		STDOFF	RULES	FORMAT	[UNTIL]
> Zone Atlantic/Azores	-1:42:40 -	LMT	1884        # Ponta Delgada
> 			-1:54:32 -	HMT	1912 Jan  1  2:00u # Horta MT
> 			-2:00	Port	-02/-01	1942 Apr 25 22:00s
> 			-2:00	Port	+00	1942 Aug 15 22:00s
> 			-2:00	Port	-02/-01	1943 Apr 17 22:00s
> 			-2:00	Port	+00	1943 Aug 28 22:00s
> 			-2:00	Port	-02/-01	1944 Apr 22 22:00s
> 			-2:00	Port	+00	1944 Aug 26 22:00s
> 			-2:00	Port	-02/-01	1945 Apr 21 22:00s
> 			-2:00	Port	+00	1945 Aug 25 22:00s
> 			-2:00	Port	-02/-01	1966 Apr  3  2:00
> 			-1:00	Port	-01/+00	1983 Sep 25  1:00s
> 			-1:00	W-Eur	-01/+00	1992 Sep 27  1:00s

the 'europe' file should contain this:

> # Zone NAME		STDOFF	RULES	FORMAT	[UNTIL]
> Zone Atlantic/Azores	-1:42:40 -	LMT	1884        # Ponta Delgada
> 			-1:54:32 -	HMT	1912 Jan  1  2:00u # Horta MT
> 			-2:00	Port	%z	1966 Apr  3  2:00
> 			-1:00	Port	%z	1983 Sep 25  1:00s
> 			-1:00	W-Eur	%z	1992 Sep 27  1:00s

This would eventually simplify future maintenance and help avoid similar
glitches in the future. For now, the 'rearguard' format would continue 
to avoid %z in the FORMAT column.

It's reasonable to use %z in 'europe' now, as zic has supported %z since 
release 2015f.