tail: ignore -f for piped-stdin, as POSIX requires

tail: ignore -f for piped-stdin, as POSIX requires

* src/tail.c (main): Tailing a pipe "forever" is not useful,
and POSIX specifies that tail ignore the -f when there is no
file argument and stdin is a FIFO or pipe.  So we do that.
In addition, GNU tail excludes "-" arguments from the list of files
to tail forever, when the associated file descriptor is connected
to a FIFO or pipe.  Before this change, ":|tail -f" would hang.
Reported by Ren Yang and Ulrich Drepper.
* tests/tail-2/pipe-f: Test for this.
* tests/tail-2/pipe-f2: Ensure tail doesn't exit early for a fifo.
* tests/Makefile.am (TESTS): Add these tests.
* NEWS (POSIX conformance): Mention it.