[tz] small bug fix and ugliness concentration

[Paul Eggert]

Christos Zoulas wrote:

> There there were 2 copies of the USG_COMPAT/ALTZONE ugliness and one
> was incomplete (did not set daylight).

It wasn't incomplete, as there is no need to set 'daylight' for a particular 
time stamp.  'daylight' is a property of the entire time zone setting, not of 
one particular time stamp using that setting.

That being said, your suggestion simplifies the code, so I fixed it up and 
applied the attached proposed patch.  I noticed a related tzalloc bug while 
looking into this, and will follow up with another proposed patch.
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