Fix handling of previous password hooks in passwordcheck

When piling up loading of modules using check_password_hook_type,
loading passwordcheck would remove any trace of a previously-loaded
hook.  Unloading the module would also cause previous hooks to be
entirely gone.

Reported-by: Rafael Castro
Author: Michael Paquier
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/15932-78f48f9ef166778c@postgresql.org
Backpatch-through: 9.4