pgsql: Don't spuriously report FD_SETSIZE exhaustion on Windows.

Don't spuriously report FD_SETSIZE exhaustion on Windows.

Starting on 2023-08-03, this intermittently terminated a "pgbench -C"
test in CI. It could affect a high-client-count "pgbench" without "-C".
While parallel reindexdb and vacuumdb reach the same problematic check,
sufficient client count and/or connection turnover is less plausible for
them. Given the lack of examples from the buildfarm or from manual
builds, reproducing this must entail rare operating system
configurations. Also correct the associated error message, which was
wrong for non-Windows. Back-patch to v12, where the pgbench check first
appeared. While v11 vacuumdb has the problematic check, reaching it
with typical vacuumdb usage is implausible.

Reviewed by Thomas Munro.

Discussion: https://postgr.es/m/CA+hUKG+JwvTNdcyJTriy9BbtzF1veSRQ=9M_ZKFn9_LqE7Kp7Q@mail.gmail.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1102f4ece30ed734ca2193fedac74395490aee99

Modified Files
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src/bin/pgbench/pgbench.c | 19 ++++++++++++++-----
src/fe_utils/parallel_slot.c | 37 +++++++++++++++++++++++++++++++++++--
2 files changed, 49 insertions(+), 7 deletions(-)