archive/zip: preserve old FileHeader.ModTime behavior

In order to avoid a regression where the date of the ModTime method
changed behavior, simply preserve the old behavior of determining
the date based on the legacy fields.

This ensures that anyone relying on ModTime before Go1.10 will have
the exact same behavior as before.
New users should use FileHeader.Modified instead.

We keep the UTC coersion logic in SetModTime since some users
manually compute timezone offsets in order to have precise control
over the MS-DOS time field.

Fixes #22738

Change-Id: Ib18b6ebd863bcf645748e083357dce9bc788cdba
Reviewed-on: https://go-review.googlesource.com/78031


Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>