[sepgsql 1/3] add name qualified creation label

This patch adds sepgsql the feature of name qualified creation label.

Background, on creation of a certain database object, sepgsql assigns
a default security label according to the security policy that has a set of
rules to determine a label of new object.
Usually, a new object inherits its parent (e.g table is a parent of column)
object's label, unless it has a particular type_transition rule in the policy.
Type_transition rule allows to describe a particular security label as
default label of new object towards a pair of client and parent object.
For example, the below rule says columns constructed under the table
labeled as "sepgsql_table_t" by client with "staff_t" will have
"staff_column_t", instead of table's label.
TYPE_TRANSITION staff_t sepgsql_table_t:db_column staff_column_t;

Recently, this rule was enhanced to take 5th argument for object name;
that enables to special case handling exceptionally.
It was originally designed to describe default security labels for files in
/etc directory, because many application put its own configuration files
here, thus, traditional type_transition rule was poor to describe all the
needed defaults.
On the other hand, we can port this concept of database system also.
One example is temporary objects being constructed under the pg_temp
schema. If we could assign a special default label on this, it allows
unprivileged users (who cannot create persistent tables) to create
temporary tables that has no risk of information leak to other users.
Otherwise, we may be able to assign a special security label on
system columns and so on.

From the perspective of implementation on sepgsql side, all we need
to do is replace old security_compute_create_raw() interface by new
security_compute_create_name_raw().
If here is no name qualified type_transition rules, it performs as if
existing API, so here is no backword compatible issue.

This patch can be applied on the latest master branch.

Thanks,
--
KaiGai Kohei <kaigai(at)kaigai(dot)gr(dot)jp>