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: /messages/by-id/af5e0bf3-6a61-bb97-6cba-061ddf22ff6b@dunslane.net

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/878f38b80e91ab2727b070fe5a0962820a09a33b

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