[tz] [PROPOSED] Rename internal identifiers to match RFC 8536

[Paul Eggert]

On 6/16/19 11:50 PM, Stephen Colebourne wrote:
> This is clearly a change to the API as seen by downstream consumers.
> Some downstream consumers may well parse the file and look for a
> column named "GMTOFF".

The "GMTOFF"-to-"DSTOFF" change affects only comments, which are clearly 
not part of the API: the comments are not present in the tzdata.zi file 
that is input to zic in the reference Makefile, and they are not 
invariably spelled the same way in the source so it's unlikely they're 
(incorrectly) being relied on by downstream consumers. We could change 
ziguard.awk to change DSTOFF to GMTOFF in the rearguard format's 
comments, and this would suffice to handle the problem if it arises, 
though I doubt that it will.