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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master

Details
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https://git.postgresql.org/pg/commitdiff/0c8910a0cab7c1e439bf5f5850122c36359e6799

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(-)