Sync our copy of the timezone library with IANA release tzcode2018e.

The non-cosmetic changes involve teaching the "zic" tzdata compiler about
negative DST.  While I'm not currently intending that we start using
negative-DST data right away, it seems possible that somebody would try
to use our copy of zic with bleeding-edge IANA data.  So we'd better be
out in front of this change code-wise, even though it doesn't matter for
the data file we're shipping.

Discussion: https://postgr.es/m/30996.1525445902@sss.pgh.pa.us