Add support for privileges on types

This adds support for the more or less SQL-conforming USAGE privilege
on types and domains.  The intent is to be able restrict which users
can create dependencies on types, which restricts the way in which
owners can alter types.

reviewed by Yeb Havinga

doc/src/sgml/ref/create_aggregate.sgml

@@ -163,6 +163,13 @@ SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
    than</quote> or <quote>greater than</quote> strategy member of a B-tree
    index operator class.
   </para>
+
+  <para>
+   To be able to create an aggregate function, you must
+   have <literal>USAGE</literal> privilege on the argument types, the state
+   type, and the return type, as well as <literal>EXECUTE</literal> privilege
+   on the transition and final functions.
+  </para>
  </refsect1>
 
  <refsect1>