Prepend an underscore (_) to functions meant to be run by superuser

_CDB_SetUp_User_PG_FDW_Server and _CDB_Drop_User_PG_FDW_Server are
meant to be executed by a superuser. Therefore they shouldn't be
considered part of the public API and hence the _CDB_Private_Function
naming convention.

scripts-available/CDB_ForeignTable.sql

@@ -170,7 +170,7 @@ LANGUAGE plpgsql VOLATILE PARALLEL UNSAFE;
 --   * Specific roles: GRANT amazon TO role_name;
 --   * Members of the organization: SELECT cartodb.CDB_Organization_Grant_Role('amazon');
 --   * The publicuser: GRANT amazon TO publicuser;
-CREATE OR REPLACE FUNCTION @extschema@.CDB_SetUp_User_PG_FDW_Server(fdw_name NAME, config json)
+CREATE OR REPLACE FUNCTION @extschema@._CDB_SetUp_User_PG_FDW_Server(fdw_name NAME, config json)
 RETURNS void AS $$
 DECLARE
   row record;
@@ -248,7 +248,7 @@ $$ LANGUAGE plpgsql VOLATILE PARALLEL UNSAFE;
 --   SELECT cartodb.CDB_Drop_User_PG_FDW_Server('amazon')
 --
 -- Note: if there's any dependent object (i.e. foreign table) this call will fail
-CREATE OR REPLACE FUNCTION @extschema@.CDB_Drop_User_PG_FDW_Server(fdw_name NAME)
+CREATE OR REPLACE FUNCTION @extschema@._CDB_Drop_User_PG_FDW_Server(fdw_name NAME)
 RETURNS void AS $$
 BEGIN
     EXECUTE FORMAT ('DROP SCHEMA %I', fdw_name);

test/extension/test.sh

@@ -607,7 +607,7 @@ test_extension|public|"local-table-with-dashes"'
    }
 }
 EOF
-    sql postgres "SELECT cartodb.CDB_SetUp_User_PG_FDW_Server('test_user_fdw', '$ufdw_config');"
+    sql postgres "SELECT cartodb._CDB_SetUp_User_PG_FDW_Server('test_user_fdw', '$ufdw_config');"
 
     # Grant a user access to that FDW, and to grant to others
     sql postgres "GRANT test_user_fdw TO cdb_testmember_1 WITH ADMIN OPTION;"
@@ -641,9 +641,9 @@ EOF
     sql cdb_testmember_1 "REVOKE test_user_fdw FROM publicuser;"
 
     # If there are dependent objects, we cannot drop the foreign server
-    sql postgres "SELECT cartodb.CDB_Drop_User_PG_FDW_Server('test_user_fdw')" fails
+    sql postgres "SELECT cartodb._CDB_Drop_User_PG_FDW_Server('test_user_fdw')" fails
     sql cdb_testmember_1 "DROP FOREIGN TABLE test_user_fdw.foo;"
-    sql postgres "SELECT cartodb.CDB_Drop_User_PG_FDW_Server('test_user_fdw')"
+    sql postgres "SELECT cartodb._CDB_Drop_User_PG_FDW_Server('test_user_fdw')"
 
 
     # Teardown