[tz] Add new timezone for Hanoi Capital, Vietnam

[Guy Harris]

On Feb 18, 2019, at 6:41 AM, Tim Parenti <tim at timtimeonline.com> wrote:

> This, however, is where the need for a second zone for Vietnam arises.  And indeed there are already two listed in zone1970.tab (Asia/Ho_Chi_Minh and Asia/Bangkok);

So Hanoi time has always had the same offset same as Bangkok time from 1970-01-01 00:00:00 UTC to the present, and, like Bangkok, has never had summer time/DST?

If so, then is the reason that we have

	Link Asia/Bangkok Asia/Phnom_Penh       # Cambodia
	Link Asia/Bangkok Asia/Vientiane        # Laos

but don't have, for example,

	Link Asia/Bangkok Asia/Hanoi            # (North) Vietnam

that Asia/Phnom_Penh and Asia/Vientiane existed as separate entries before being combined with Asia/Bangkok, so that we need to keep their entire around for backwards compatibility, but there had never been an Asia/Hanoi?