pgsql: Use WL_SOCKET_CLOSED for client_connection_check_interval.

Use WL_SOCKET_CLOSED for client_connection_check_interval.

Previously we used poll() directly to check for a POLLRDHUP event.
Instead, use the WaitEventSet API to poll the socket for
WL_SOCKET_CLOSED, which knows how to detect this condition on many more
operating systems.

Reviewed-by: Zhihong Yu <zyu(at)yugabyte(dot)com>
Reviewed-by: Maksim Milyutin <milyutinma(at)gmail(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/77def86b27e41f0efcba411460e929ae%40postgrespro.ru

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/cba5b994c990bba8df9b8bb75f25ca40aef6b68b

Modified Files
--------------
doc/src/sgml/config.sgml | 6 +++---
src/backend/libpq/pqcomm.c | 46 ++++++++++++++++++++++----------------------
src/backend/utils/misc/guc.c | 7 ++-----
src/include/libpq/libpq.h | 1 +
4 files changed, 29 insertions(+), 31 deletions(-)