Distinguishing autovacuum activity in pg_stat_activity

I was just noticing that $SUBJECT is hard --- the entry in current_query
looks exactly like a manual vacuum command, and there's not anything in
the other fields that looks different either.

Since autovacuum.c is making up its pgstat_report_activity string
anyway, it would be easy to make the string look like, say,AUTO VACUUM ANALYZE foo.bar

I think this would be a helpful change, but some might find it
confusing.  Thoughts?
        regards, tom lane