Teach relation_is_updatable() about partitioned tables.

Table partitioning, introduced in commit f0e44751, added a new
relkind - RELKIND_PARTITIONED_TABLE. Update relation_is_updatable() to
handle it. Specifically, partitioned tables and simple views built on
top of them are updatable.

This affects the SQL-callable functions pg_relation_is_updatable() and
pg_column_is_updatable(), and the views information_schema.views and
information_schema.columns.

Dean Rasheed, reviewed by Ashutosh Bapat.

Discussion: https://postgr.es/m/CAEZATCXnbiFkMXgF4Ez1pmM2c-tS1z33bSq7OGbw7QQhHov%2B6Q%40mail.gmail.com