tail: always fail when followed files become inaccessible

tail: always fail when followed files become inaccessible

* src/tail.c (tail_forever): Without --retry, exit with failure
status like we do for the inotify case (since v8.11-15-g61de57cd2).
This is also consistent with the failure exit if no file was
accessible at tail startup.
* tests/tail/follow-stdin.sh: Tweak due to earlier exit.
* tests/tail/follow-name.sh: Test with and without inotify.
* NEWS: Mention the bug fix.