pgsql: Allow parent's WaitEventSets to be freed after fork().

Allow parent's WaitEventSets to be freed after fork().

An epoll fd belonging to the parent should be closed in the child.  A
kqueue fd is automatically closed by fork(), but we should still adjust
our counter.  For poll and Windows systems, nothing special is required.
On all systems we free the memory.

No caller yet, but we'll need this if we start using WaitEventSet in the
postmaster as planned.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKG%2BZ-HpOj1JsO9eWUP%2Bar7npSVinsC_npxSy%2BjdOMsx%3DGg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b5d0f8ec01c021452203b2fd3921c9b55f6c3cd3

Modified Files
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src/backend/storage/ipc/latch.c | 17 +++++++++++++++++
src/include/storage/latch.h     |  1 +
2 files changed, 18 insertions(+)