tail: terminate when following pipes and untailable non pipes

tail: terminate when following pipes and untailable non pipes

* src/tail.c (ignore_pipe_or_fifo): Mark the descriptor as -1
for pipes so that any_live_files() detects correctly that
the entry is no longer live.
* tests/tail-2/pipe-f.sh: Add a test case.
* NEWS: Mention the fix.
Fixes http://bugs.gnu.org/24903 which was detected
using Symbolic Execution techniques developed in
the course of the SYMBIOSYS research project at
COMSYS, RWTH Aachen University.