tail: exit promptly when output no longer writable

tail: exit promptly when output no longer writable

This will support use cases like:

  tail -f file.log | grep -q trigger &&
  process_immediately

* src/tail.c (check_output_alive): A new function that
uses select on fifos or pipes to detect if they're broken.
(tail_forever): Call check_output_alive() periodically.
(tail_forever_inotify): Merge the select() call from
check_output_alive() into the select() originally present
for the --pid case, and adjust accordingly.
* tests/tail-2/pipe-f.sh: Add test cases.
* NEWS: Mention the improvement.