pgsql: On Windows, also call shutdown() while closing the client socket

On Windows, also call shutdown() while closing the client socket.

Further experimentation shows that commit 6051857fc is not sufficient
when using (some versions of?) OpenSSL.  The reason is obscure, but
calling shutdown(socket, SD_SEND) improves matters.

Per testing by Andrew Dunstan and Alexander Lakhin.
Back-patch as before.

Discussion: https://postgr.es/m/af5e0bf3-6a61-bb97-6cba-061ddf22ff6b@dunslane.net

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a8a983e8299ae7e63b18b3965ca1f55e18f84a06

Modified Files
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src/backend/libpq/pqcomm.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)