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: https://postgr.es/m/3369130.1649348542@sss.pgh.pa.us