MFC r342613: Import tzdata 2018h
MFC r342644: Import tzdata 2018i

MFC r339848: Import tzdata 2018g

MFC r333247: Import tzdata 2018e

North Korea switches back to +09 on 2018-05-05.

This version more correctly models time stamps in time zones with
negative DST such as Europe/Dublin (from 1971 on), Europe/Prague
(1946/7), and Africa/Windhoek (1994/2017). This does not affect the
UT offsets, only time zone abbreviations and the tm_isdst flag.

MFC r331481: Import tzdata 2018d