Emit a warning if the extension's GUC is set incorrectly

Hi hackers,

If wrong GUCs of auth_delay, pg_trgm, postgres_fdw and sepgsql are 
described in postgresql.conf, a warning is not emitted unlike 
pg_stat_statements, auto_explain and pg_prewarm.
So, I improved it by adding EmitWarningsOnPlaceholders.
An example output is shown below.
---
2021-11-14 18:18:16.486 JST [487067] WARNING:  unrecognized 
configuration parameter "auth_delay.xxx"
---

What do you think?

-- 
Regards,

--
Shinya Kato
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION