In particular, left join to pg_authid so that rows in pg_stat_activity
don't disappear if the session's owning user has been dropped.
Also convert a few joins to pg_database to left joins, in the same spirit,
though that case might be harder to hit.  We were doing this in other
views already, so it was a bit inconsistent that these views didn't.

Oskari Saarenmaa, with some further tweaking by me

Discussion: <56E87CD8.60007@ohmu.fi>