Revert "journal: Make sd_journal_previous/next() return 0 at HEAD/TAIL"
This commit broke backwards compatibility so let's revert it until
we find a better solution.
This reverts commit 977ad21b5b8f6323515297bd8995dcaaca0905df.
(cherry picked from commit 1db6dbb1dcdacfd7d2b4c84562fc6e77bc8c43a5)