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_function.sgml

@@ -92,6 +92,11 @@ CREATE [ OR REPLACE ] FUNCTION
   <para>
    The user that creates the function becomes the owner of the function.
   </para>
+
+  <para>
+   To be able to create a function, you must have <literal>USAGE</literal>
+   privilege on the argument types and the return type.
+  </para>
  </refsect1>
 
  <refsect1>