[tz] Add new timezone for Hanoi Capital, Vietnam

[Paul Eggert]

On 2/18/19 7:56 AM, Michael H Deckers via tz wrote:
> this is the only case where there
>    is a name in backzone/ that does not occur as a link
>    in the remaining data. 

Yes, that's correct. The 'backzone' file mostly holds data I created 
long ago that was later determined to be out of scope for tzdb, and so 
moved to 'backzone' with Link lines left behind for 
backward-compatibility reasons. Once that was done, there has been 
little motivation to create more 'backzone' entries, they're out of 
scope. The only exception so far has been Hanoi, as I later got 
high-quality data for Vietnam from Trần's book that was useful for 
improving Asia/Ho_Chi_Minh, and puzzling the data out gave an entry for 
Hanoi that seemed worth recording in 'backzone' even though it's out of 
scope. This exception doesn't need a backward-compatibility link, since 
there's nothing to be backward-compatible to.