support for version-3 format tz binary files

[Paul Eggert]

Joseph S. Myers wrote:
> I am operating under the presumption the files in timezone/ that (as per 
> the README there) come from tzcode should not be updated until this patch 
> series is in.

I wouldn't presume that, as the current (unpatched) libc/time/tzset.c
should work just fine with files generated by the latest (2013g) tz code,
except for a few cases far in the future.  The earliest affected time stamp
is America/Godthab's 2038-03-28 01:00:00 UTC, which is after the 32-bit signed
time_t values have rolled around.