Re: Updating our timezone code in the back branches

When I updated our copy of the IANA timezone library back in March
(commit 1c1a7cbd6), I noted that we ought to consider back-patching
those changes once they'd settled out in HEAD.  Now that the code
has survived a couple of months of beta testing, I think it is time
to do that.

I went through the IANA announcement archives
http://mm.icann.org/pipermail/tz-announce/
to get a summary of what's changed since the tzcode 2010c release
that we last synced with.  Attached is a list of code changes that
seem potentially significant to us.  There are at least two ticking
time bombs in there, namely zic feature additions that IANA has not
yet started to use in the tzdata files, but presumably will start
using at some point, else why would they put them in?  (These are
the extension to allow four DST transitions per year and the addition
of %z escapes in TZ abbreviations.)  Whenever they do start using them,
we'll be unable to apply tzdata updates to unpatched back branches,
because our old copy of zic will fail to compile the tzdata files.

There are also several bug fixes that affect interpretation of dates after
2037, a year that's getting closer all the time.  And there are some
changes that affect reading of zic data files, which could affect
unpatched back branches that are built with --with-system-tzdata,
since those might be fed updated data files even if we do nothing.

So I think it behooves us to apply these changes to the back branches
while we can still do it in a leisurely fashion, rather than waiting
until our hands are forced.  I'd like to do it in the next week or so
so that we can get in a little bit of buildfarm and manual testing
before the next set of back-branch releases in August.

Thoughts, objections?