pgsql: Rename hook functions for debug_io_direct to match variable name

Rename hook functions for debug_io_direct to match variable name.

Commit 319bae9a renamed the GUC.  Rename the check and assign functions
to match, and alphabetize.

Back-patch to 16.

Author: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/2769341e-fa28-c2ee-3e4b-53fdcaaf2271%40eisentraut.org

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/155c81463c26d908a1579b6d90e7b39b6f61df8b
Author: Peter Eisentraut <peter@eisentraut.org>

Modified Files
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src/backend/storage/file/fd.c       | 6 +++---
src/backend/utils/misc/guc_tables.c | 6 +++---
src/include/utils/guc_hooks.h       | 4 ++--
3 files changed, 8 insertions(+), 8 deletions(-)