pgsql: Modernize some for loops

Modernize some for loops

in guc-related source files, in anticipation of some further
restructuring.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/8fdfb91e-60fb-44fa-8df6-f5dea47353c9@eisentraut.org

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/1a795188889cc630f4f9ce617bfc28fb2aa1432f

Modified Files
--------------
src/backend/utils/misc/guc.c         | 135 +++++++++++++----------------------
src/backend/utils/misc/guc_funcs.c   |   3 +-
src/backend/utils/misc/help_config.c |   5 +-
3 files changed, 53 insertions(+), 90 deletions(-)