[tz] %z in main format (was: Portugal)

[Paul Eggert]

This 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:

>                         -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:

>                        -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.

It's reasonable to move %z to main format now, as zic has supported %z 
since release 2015f.