Fix sepgsql regression tests (9.2-only patch).

The regression tests for sepgsql were broken by changes in the
base distro as-shipped policies. Specifically, definition of
unconfined_t in the system default policy was changed to bypass
multi-category rules, which the regression test depended on.
Fix that by defining a custom privileged domain
(sepgsql_regtest_superuser_t) and using it instead of system's
unconfined_t domain. The new sepgsql_regtest_superuser_t domain
performs almost like the current unconfined_t, but restricted by
multi-category policy as the traditional unconfined_t was.

The custom policy module is a self defined domain, and so should not
be affected by related future system policy changes. However, it still
uses the unconfined_u:unconfined_r pair for selinux-user and role.
Those definitions have not been changed for several years and seem
less risky to rely on than the unconfined_t domain. Additionally, if
we define custom user/role, they would need to be manually defined
at the operating system level, adding more complexity to an already
non-standard and complex regression test.

Applies only to 9.2. Unlike the previous similar patch, commit 794e2558,
this also fixes a bug related to processing SELECT INTO statement.
Because v9.2 didn't have ObjectAccessPostCreate to inform the context
when a relation is newly created, sepgsql had an alternative method.
However, related code in sepgsql_object_access() neglected to consider
T_CreateTableAsStmt, thus no label was assigned on the new relation.
This logic was removed and replaced starting in 9.3.

Patch by Kohei KaiGai.