pgsql: Fix case sensitivity in psql's tab completion for GUC names.

Fix case sensitivity in psql's tab completion for GUC names.

Input for these should be case-insensitive, but was not completely
so. Comparing to the similar queries for timezone names, I realized
that we'd missed forcing the comparison pattern to lower-case.
With that, it behaves as I expect.

While here, flatten the sub-selects in these queries; I don't
find that those add any readability.

Discussion: /messages/by-id/3369130.1649348542@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b5607b0746f40b3f5b38004c64ccc9697ee1e222

Modified Files
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src/bin/psql/tab-complete.c | 22 ++++++++--------------
1 file changed, 8 insertions(+), 14 deletions(-)