The timezone map tries to highlight regions of the world that keep the
same time, approximating this by their non-daylight-savings UTC offset.
There's no reasonable API for this, and it goes wrong in various cases,
such as Irish Standard Time which is legally defined as the country's
summer time with a negative DST offset in winter.
Hardcoding this is unpleasant, but there doesn't seem to be a better
solution, and in any case there's already similar hardcoding implied by
the segmented map images in panels/datetime/data/timezone_*.png. I've
tried to make it practical to fix other similar disagreements between
the detected offset and the groupings implied by map images, though for
now I've conservatively fixed only the case I'm familiar with.
Fixes: #1341
The code that set it could only ever have worked by luck in some
timezones on some systems, and there's no API to obtain the information
we were trying to store in it. Nothing uses it, so there's no point
trying to fix it.
tzname_normal is also unused, but slightly useful for debugging, so
rename it to tzname but otherwise keep it in place.
tz_info_from_location() was changing the value of TZ, but not
restoring it when it was done checking data for it, and setting
the GMT timezone instead.
This meant that UTC time was displayed when the minute changed on
the wall clock.
https://bugzilla.gnome.org/show_bug.cgi?id=649568
