Fix our mapping of Windows timezones for Central America.

Fix our mapping of Windows timezones for Central America.

We were mapping "Central America Standard Time" to "CST6CDT", which seems
entirely wrong, because according to the Olson timezone database noplace
in Central America observes daylight savings time on any regular basis ---
and certainly not according to the USA DST rules that are implied by
"CST6CDT".  (Mexico is an exception, but they can be disregarded since
they have a separate timezone name in Windows.)  So, map this zone name to
plain "CST6", which will provide a fixed UTC offset.

As written, this patch will also result in mapping "Central America
Daylight Time" to CST6.  I considered hacking things so that would still
map to CST6CDT, but it seems it would confuse win32tzlist.pl to put those
two names in separate entries.  Since there's little evidence that any
such zone name is used in the wild, much less that CST6CDT would be a good
match for it, I'm not too worried about what we do with it.

Per complaint from Pratik Chirania.