Update timezone code from tzcode 2017b

[Paul Eggert]

Thanks for doing all that. Those changes look good to me.

The intent is that glibc should be insulated from tzcode changes, in 
that glibc should be able to copy the latest source code from tzcode 
unaltered, without having to change any other source file in glibc. 
Obviously this doesn't work in areas where glibc assumes particular 
contents of timezone/private.h, an assumption that is unfortunately 
necessary in a few places. For what it's worth, looking at the planned 
(i.e., post-2017b) changes to tzcode's private.h, I don't see anything 
likely to cause problems in the next merge. If you can see any changes 
to the tzcode source files that could help avoid similar merge problems 
in the future, they'd be welcome, as I want the merge process to be as 
painless as practical.