pgsql: Teach SHOW ALL to honor pg_read_all_settings membership

Teach SHOW ALL to honor pg_read_all_settings membership

Also, fix the pg_settings view to display source filename and line
number when invoked by a pg_read_all_settings member. This addition by
me (Álvaro).

Also, fix wording of the comment in GetConfigOption regarding the
restriction it implements, renaming the parameter for extra clarity.
Noted by Michaël.

These were all oversight in commit 25fff40798fc; backpatch to pg10,
where that commit first appeared.

Author: Laurenz Albe
Reviewed-by: Michaël Paquier, Álvaro Herrera
Discussion: https://postgr.es/m/1519917758.6586.8.camel@cybertec.at

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a25c207b2fb8be8dc1bf91b55e8b5b3d809934d9

Modified Files
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src/backend/utils/misc/guc.c | 19 ++++++++++---------
src/include/utils/guc.h | 2 +-
2 files changed, 11 insertions(+), 10 deletions(-)