Add PROCESS_MAIN to VACUUM

Disabling this option is useful to run VACUUM (with or without FULL) on
only the toast table of a relation, bypassing the main relation.  This
option is enabled by default.

Running directly VACUUM on a toast table was already possible without
this feature, by using the non-deterministic name of a toast relation
(as of pg_toast.pg_toast_N, where N would be the OID of the parent
relation) in the VACUUM command, and it required a scan of pg_class to
know the name of the toast table.  So this feature is basically a
shortcut to be able to run VACUUM or VACUUM FULL on a toast relation,
using only the name of the parent relation.

A new switch called --no-process-main is added to vacuumdb, to work as
an equivalent of PROCESS_MAIN.

Regression tests are added to cover VACUUM and VACUUM FULL, looking at
pg_stat_all_tables.vacuum_count to see how many vacuums have run on
each table, main or toast.

Author: Nathan Bossart
Reviewed-by: Masahiko Sawada
Discussion: https://postgr.es/m/20221230000028.GA435655@nathanxps13

src/backend/postmaster/autovacuum.c

@@ -2860,7 +2860,9 @@ table_recheck_autovac(Oid relid, HTAB *table_toast_map,
 		 * skip vac_update_datfrozenxid(); we'll do that separately.
 		 */
 		tab->at_params.options =
-			(dovacuum ? (VACOPT_VACUUM | VACOPT_SKIP_DATABASE_STATS) : 0) |
+			(dovacuum ? (VACOPT_VACUUM |
+						 VACOPT_PROCESS_MAIN |
+						 VACOPT_SKIP_DATABASE_STATS) : 0) |
 			(doanalyze ? VACOPT_ANALYZE : 0) |
 			(!wraparound ? VACOPT_SKIP_LOCKED : 0);