In 3.0, ds_mark_dst() had its parameters swapped by mistake, without any
real benefit in doing so, both breaking backwards-compatibility and also
leading to uglier script, such as:
This commit simply reverts the function to its previous behavior,
addressing both issues above. Hopefully, I don't upset too many people
who migrated to 3.0 during this month following the stable release.
(cherry picked from commit 11884b2375a97ad9d7b0cef7322ad73d2d6bbff8)