Add LOG_NEVER error level code.

This logging level means not to emit the log, which is useful for
functions like relation_needs_vacanalyze().  This function accepts
a log level argument but not all callers want it to emit logs.

Suggested-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3101163.1775676098%40sss.pgh.pa.us

src/backend/postmaster/autovacuum.c

@@ -3676,7 +3676,8 @@ pg_stat_get_autovacuum_scores(PG_FUNCTION_ARGS)
 
 		avopts = extract_autovac_opts(tup, RelationGetDescr(rel));
 		relation_needs_vacanalyze(form->oid, avopts, form,
-								  effective_multixact_freeze_max_age, 0,
+								  effective_multixact_freeze_max_age,
+								  LOG_NEVER,
 								  &dovacuum, &doanalyze, &wraparound,
 								  &scores);
 		if (avopts)

src/include/utils/elog.h

@@ -23,6 +23,7 @@ struct Node;
 
 
 /* Error level codes */
+#define LOG_NEVER	0			/* Never emit this message */
 #define DEBUG5		10			/* Debugging messages, in categories of
 								 * decreasing detail. */
 #define DEBUG4		11