show "aggressive" or not in autovacuum logs

Hello, it would be too late but I'd like to propose this because
this cannot be back-patched.

In autovacuum logs, "%u skipped frozen" shows the number of pages
skipped by ALL_FROZEN only in aggressive vacuum.

So users cannot tell whether '0 skipped-frozen' means a
non-agressive vacuum or no frozen-pages in an agressive vacuum.

I think it is nice to have an indication whether the scan was
"agressive" or not in log output. Like this,

> LOG: automatic aggressive vacuum of table "template1.pg_catalog.pg_statistic": index scans: 0

"0 skipped frozen" is uesless in non-aggressive vacuum but
removing it would be too-much. Inserting "aggressive" reduces
machine-readability so it might be better in another place. The
attached patch does the following.

> LOG: automatic vacuum of table "postgres.public.pgbench_branches": mode: normal, index scans: 0
> LOG: automatic vacuum of table "postgres.public.pgbench_branches": mode: aggressive, index scans: 0

regards,

--
Kyotaro Horiguchi
NTT Open Source Software Center