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