Update timezone to C99

The code in src/timezone/ still contained workarounds for not wanting to
rely on C99 <stdint.h> and <inttypes.h>, even though the rest of the
code uses those now.

To fix that, I re-downloaded the upstream code (same version as before),
applied the fixes described in src/timezone/README, except those related
to the integer types, and then put back the PostgreSQL-specific code and
massaged things to remove irrelevant differences. There were a few
other minor and cosmetic changes that I left in to improve alignment
with upstream.

Patch attached. Seems to work.