Loading and testing timezones is currently implemented using several,
partly redundant, OS specific data structures and functions. This
change merges most of that code into OS independent implementations.

Change-Id: Iae2877c5f48d1e4a9de9ce55d0530d52e24cf96e
Reviewed-on: https://go-review.googlesource.com/64391
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>