tail: fix tailing larger number of lines in regular files

tail: fix tailing larger number of lines in regular files

* src/tail.c (file_lines): Seek to the previous block instead of the
beginning (or a little before) of the block that was just scanned.
Otherwise, the same block is read and scanned (at least partially)
again. This bug was introduced by commit v9.7-219-g976f8abc1.
* tests/tail/basic-seek.sh: Add a new test.
* tests/local.mk: Reference the new test.
* NEWS: mention the bug fix.