pgsql: Fix initdb's -c option to treat the GUC name case-insensitively.

Fix initdb's -c option to treat the GUC name case-insensitively.

The backend treats GUC names case-insensitively, so this code should
too.  This avoids ending up with a confusing set of redundant entries
in the generated postgresql.conf file.

Per report from Kyotaro Horiguchi.  Back-patch to v16 where this
feature was added (in commit 3e51b278d).

Discussion: https://postgr.es/m/20230928.164904.2153358973162534034.horikyota.ntt@gmail.com

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b78f4d22b2f21a3efd1cb8969c9e8fa1f52466c5

Modified Files
--------------
src/bin/initdb/initdb.c        |  9 +++++++--
src/bin/initdb/t/001_initdb.pl | 14 ++++++++++++++
2 files changed, 21 insertions(+), 2 deletions(-)