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 25fff407; 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