Rename EmitWarningsOnPlaceholders() to MarkGUCPrefixReserved().

This seems like a clearer name for what it does now.

Provide a compatibility macro so that extensions don't have to convert
to the new name right away.

Discussion: https://postgr.es/m/116024.1640111629@sss.pgh.pa.us

contrib/sepgsql/hooks.c

@@ -455,7 +455,7 @@ _PG_init(void)
 							 NULL,
 							 NULL);
 
-	EmitWarningsOnPlaceholders("sepgsql");
+	MarkGUCPrefixReserved("sepgsql");
 
 	/* Initialize userspace access vector cache */
 	sepgsql_avc_init();