[tz] Split America into North America and South America

[Paul Eggert]

I chose "America/" in the 1990s to keep names shorter and avoid worrying 
about borderline cases - e.g., should it be 
"North_America/Port_of_Spain" or "South_America/Port_of_Spain"?

Now that we've moved borderline cases like America/Curacao, 
America/Grenada, and America/Port_of_Spain to 'backward' the second 
justification has less force, at least until the southern Caribbean 
starts messing with its clocks. However, this also means America/* is 
less populated by unique entries identified by zone1970.tab.  Currently 
zone1970.tab lists 95 Zones directly under America and 75 under Asia, 
hardly the order-of-magnitude difference that might necessitate the 
disruption of a great renaming.

As Guy notes, users should not do 'ls /usr/share/zoneinfo/America' 
anyway; they should use a timezone chooser. Even the primitive chooser 
tzselect lists at most 54 names at once (all the countries in Asia, 
which as it happens outnumbers all the countries in North and South 
America combined). And commonly used timezone choosers let you point at 
a map, or figure things out from your location, or whatever. If we want 
to simplify timezone choosing we should be focusing our efforts there.