bug#21020: 24.4; `display-time-world' tampers with TZ

[Paul Eggert]

Thanks for the bug report.  Improving Emacs time zones has been on my TODO list 
for ages, and the report prompted me to do that.  Attached is a patch which 
works for me on GNU/Linux.  I'm CC'ing this to Eli to give him a heads-up in 
case it has implications on MS-Windows.

0001-New-optional-ZONE-arg-for-format-time-string-etc.txt
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