pgsql: Rename io_direct to debug_io_direct.

Rename io_direct to debug_io_direct.

Give the new GUC introduced by d4e71df6 a name that is clearly not
intended for mainstream use quite yet.

Future proposals would drop the prefix only after adding infrastructure
to make it efficient. Having the switch in the tree sooner is good
because it might lead to new discoveries about the hazards awaiting us
on a wide range of systems, but that name was too enticing and could
lead to cross-version confusion in future, per complaints from Noah and
Justin.

Suggested-by: Noah Misch <noah(at)leadboat(dot)com>
Reviewed-by: Noah Misch <noah(at)leadboat(dot)com>
Reviewed-by: Justin Pryzby <pryzby(at)telsasoft(dot)com> (the idea, not the patch)
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> (ditto)
Discussion: https://postgr.es/m/20230430041106.GA2268796%40rfd.leadboat.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/319bae9a8da66678d80d862440e54cebc7b1cc24

Modified Files
--------------
doc/src/sgml/config.sgml | 64 +++++++++++++--------------
src/backend/storage/file/fd.c | 8 ++--
src/backend/utils/misc/guc_tables.c | 2 +-
src/test/modules/test_misc/t/004_io_direct.pl | 2 +-
4 files changed, 38 insertions(+), 38 deletions(-)