support for version-3 format tz binary files

[Paul Eggert]

In the next four messages I'll send patches to document the
existing glibc support for the new version-3-format tz binary files,
and to add support for one version-3 feature that's currently missing.
I haven't tested these changes, unfortunately, as I understand it's
hard to build and install glibc, but the patches should be
straightforward.

This is all following up to bug#15968.

https://sourceware.org/bugzilla/show_bug.cgi?id=15968