pgsql: Rethink handling of settings with a prefix reserved by an extens

Rethink handling of settings with a prefix reserved by an extension.

Commit 75d22069e made SET print a warning if you tried to set an
unrecognized parameter within namespace previously reserved by an
extension. It seems better for that to be an outright error though,
for the same reason that we don't let you set unrecognized unqualified
parameter names. In any case, the preceding implementation was
inefficient and erroneous. Perform the check in a more appropriate
spot, and be more careful about prefix-match cases.

Discussion: https://postgr.es/m/116024.1640111629@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2ed8a8cc5b634d33ea07d681c6b02213da07f792

Modified Files
--------------
src/backend/utils/misc/guc.c | 96 +++++++++++++++++----------------------
src/test/regress/expected/guc.out | 36 +++++++--------
src/test/regress/sql/guc.sql | 20 ++++----
3 files changed, 68 insertions(+), 84 deletions(-)