When user= is used without password=, use userlist password.
When user= is used without password=, use userlist password.

Otherwise it will get unobvious auth failure when trying
to log in with empty password.

Main user list is consulted on connect time, because
copying/linking on load time would be hard to keep
in sync with auth file changes.

Reported by Johnny Tan

dbaxa/pgbouncer,jaiminpan/pgbouncer,wurenny/pgbouncer-x2,dbaxa/pgbouncer,digoal/pgbouncer-x2,kvap/pgbouncer,kvap/pgbouncer,jaiminpan/pgbouncer,kvap/pgbouncer,jaiminpan/pgbouncer,digoal/pgbouncer-x2,wurenny/pgbouncer-x2,digoal/pgbouncer-x2,dbaxa/pgbouncer,kvap/pgbouncer,jaiminpan/pgbouncer,wurenny/pgbouncer-x2,digoal/pgbouncer-x2,dbaxa/pgbouncer,wurenny/pgbouncer-x2