tzcode resync

[Yaakov Selkowitz]

Our winsup/cygwin/localtime.cc is a mashup of a very old version of
upstream tzcode's private.h, tzfile.h, and localtime.c with Cygwin-
specific patches for getting the timezone from windows as a fallback,
as well as handling API compatibility with pre-tm_gmtoff binaries.

The upstream tzcode/tzdata have introduced database changes which
cannot be read by old code.  Currently, I am building tzdata in the old
("rearguard") format to keep compatibility, but at this point we're
*way* behind upstream and we really should update our code.  Given how
long it's been though, it's going to be a bit of work.

Is anyone willing to take on merging our changes into the latest
upstream code and creating a new localtime.cc?  As a bonus, if the file
layout were to be restructured to make this more easily maintainable
(e.g. by creating a winsup/cygwin/tzcode subdirectory, adding
$(srcdir)/tzcode to winsup/cygwin/Makefile.in:VPATH, and updating the
object file names), that would probably help going forward.

TIA,

--
Yaakov