[v9.1] Add security hook on initialization of instance

The attached patch tries to add one more security hook on the
initialization of PostgreSQL instance (InitPostgres()).

It gives the external security module a chance to initialize itself,
and acquire credential of the client.

I assumed the best place to initialize the module is just after the
initialize_acl() invocation, if ESP is available.
We have not discussed about this hook yet. So, I'd like to see any
comments.

Thanks,
--
KaiGai Kohei <kaigai@ak.jp.nec.com>