Modify pg_dumpall so that output script uses new OWNER option of CREATE

DATABASE; also make it use SET SESSION AUTHORIZATION commands rather than
\connect commands.  This makes it possible to restore databases belonging
to users who do not have CREATEDB privilege.  It should also become at
least somewhat feasible to run the restore script under password
authentication --- you'll get one superuser password prompt per database,
rather than a large number of challenges for passwords belonging to
varying unspecified user names.