Support toasting of shared system relations, and provide toast tables for

pg_database, pg_shadow, pg_group, all of which now have potentially-long
fields.  Along the way, get rid of SharedSystemRelationNames list: shared
rels are now identified in their include/pg_catalog/*.h files by a
BKI_SHARED_RELATION macro, while indexes and toast rels inherit sharedness
automatically from their parent table.  Fix some bugs with failure to detoast
pg_group.grolist during ALTER GROUP.