postgres_fdw: Fix bug in connection status check.

postgres_fdw: Fix bug in connection status check.

The buildfarm member "hake" reported a failure in the regression test
added by commit 857df3cef7, where postgres_fdw_get_connections(true)
returned unexpected results.

The function postgres_fdw_get_connections(true) checks
if a connection is closed by using POLLRDHUP in the requested events
and calling poll(). Previously, the function only considered
POLLRDHUP or 0 as valid returned events. However, poll() can also
return POLLHUP, POLLERR, and/or POLLNVAL. So if any of these events
were returned, postgres_fdw_get_connections(true) would report
incorrect results. postgres_fdw_get_connections(true) failed to
account for these return events.

This commit updates postgres_fdw_get_connections(true) to correctly
report a closed connection when poll() returns not only POLLRDHUP
but also POLLHUP, POLLERR, or POLLNVAL.

Discussion: https://postgr.es/m/fd8f6186-9e1e-4b9a-92c5-e71e3697d381@oss.nttdata.com