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
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/4f3514f201cf007a3e0c0abbf21b70a711269ab0
Author: Peter Eisentraut <peter@eisentraut.org>

Modified Files
--------------
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(-)