pgsql: Minor GUC code refactoring.

Minor GUC code refactoring.

Split out "ConfigOptionIsVisible" to perform the privilege
check for GUC_SUPERUSER_ONLY GUCs (which these days can also
be read by pg_read_all_settings role members), and move the
should-we-show-it checks from GetConfigOptionValues to its
sole caller.

This commit also removes get_explain_guc_options's check of
GUC_NO_SHOW_ALL, which seems to have got cargo-culted in there.
While there's no obvious use-case for marking a GUC both
GUC_EXPLAIN and GUC_NO_SHOW_ALL, if it were set up that way
one would expect EXPLAIN to show it --- if that's not what
you want, then don't set GUC_EXPLAIN.

In passing, simplify the loop logic in show_all_settings.

Nitin Jadhav, Bharath Rupireddy, Tom Lane

Discussion: https://postgr.es/m/CAMm1aWYgfekpRK-Jz5=pM_bV+Om=ktGq1vxTZ_dr1Z6MV-qokA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e4e89eb5bbfdae30349b38344e9c604411174f6b

Modified Files
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src/backend/utils/misc/guc.c       | 13 +++----
src/backend/utils/misc/guc_funcs.c | 70 ++++++++++++++++++--------------------
src/include/utils/guc_tables.h     |  3 ++
3 files changed, 40 insertions(+), 46 deletions(-)