pgsql: Use FD_CLOEXEC on ListenSockets

Use FD_CLOEXEC on ListenSockets

It's good hygiene if e.g. an extension launches a subprogram when
being loaded. We went through some effort to close them in the child
process in EXEC_BACKEND mode, but it's better to not hand them down to
the child process in the first place. We still need to close them
after fork when !EXEC_BACKEND, but it's a little simpler.

In the passing, LOG a message if closing the client connection or
listen socket fails. Shouldn't happen, but if it does, would be nice
to know.

Reviewed-by: Tristan Partin, Andres Freund, Thomas Munro
Discussion: https://www.postgresql.org/message-id/7a59b073-5b5b-151e-7ed3-8b01ff7ce9ef@iki.fi

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/b0bea38705b2d14e57d353a701ecfdb8a04983b1

Modified Files
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src/backend/libpq/pqcomm.c | 6 +++++-
src/backend/postmaster/postmaster.c | 14 ++++++--------
2 files changed, 11 insertions(+), 9 deletions(-)