Revise GUC names quoting in messages again

Revise GUC names quoting in messages again

After further review, we want to move in the direction of always
quoting GUC names in error messages, rather than the previous (PG16)
wildly mixed practice or the intermittent (mid-PG17) idea of doing
this depending on how possibly confusing the GUC name is.

This commit applies appropriate quotes to (almost?) all mentions of
GUC names in error messages.  It partially supersedes a243569bf65 and
8d9978a7176, which had moved things a bit in the opposite direction
but which then were abandoned in a partial state.

Author: Peter Smith <smithpb2250@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAHut%2BPv-kSN8SkxSdoHano_wPubqcg5789ejhCDZAcLFceBR-w%40mail.gmail.com