System username in pg_stat_activity

The attached patch adds a column "authuser" to pg_stat_activity which
contains the username of the externally authenticated user, being the
same value as the SYSTEM_USER keyword returns in a backend.

This overlaps with for example the values in pg_stat_gss, but it will
include values for authentication methods that don't have their own
view such as peer/ident. gss/ssl info will of course still be shown,
it is just in more than one place.

I was originally thinking this column should be "sysuser" to map to
the keyword, but since we already have "usesysid" as a column name in
pg_stat_activity I figured that could be confusing since it actually
means something completely different. But happy to change that back if
people think that's better.

--
Magnus Hagander
Me: https://www.hagander.net/
Work: https://www.redpill-linpro.com/