[tz] Asia/Shanghai timezone history and other information

[Paul Eggert]

Thanks for that detailed info about Shanghai and environments. I installed into 
the development repository the attached proposed patch, which adds a DST period 
in Shanghai from April 13 00:00 to October 1 00:00, plus adds your commentary 
about the confusing situation in the area at the time. This patch also adds 
confirming info from the English-language Returns of Trade and Trade Reports, 1919.
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