tail: display file headers correctly with inotify

tail: display file headers correctly with inotify

* src/tail.c (tail_forever_inotify): Use the fspec pointer to
distinguish previously output files, rather than a descriptor
from the inotify event.  That event descriptor was that of
the parent directory when files were created or renamed etc.
(check_fspec): Adjust for the new comparison.  Also show the
header when the file is truncated, since we show data
in this case also.
* tests/tail-2/F-headers.sh: A new test case.
* tests/local.mk: Reference the new test.
* NEWS: Mention the bug fix.