estream: Fix gpgrt_poll at EOF.

* src/estream.c [HAVE_POLL_H] (_gpgrt_poll): Catch POLLHUP event
for want_read.

At EOF, here is a difference of semantics between poll(2) and
select(2). For poll(2), detection of EOF is an event of POLLHUP, not
POLLIN.

• Fixes-commit: a21a7de8c2cf986235382e7e04805744f6df116e
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>