tail: detect closed stdout on Solaris

tail: detect closed stdout on Solaris

* src/tail.c (check_output_alive): Use poll() on Solaris.
Also handle POLLHUP, which Solaris returns in this case.
* tests/tail-2/pipe-f.sh: Use `head -n2` rather than `sed 2q`
as Solaris sed does not exit in this case.
* NEWS: Mention the improvement.
Reported by Bruno Haible.