allow access to ns_process_registry:register_pid by the registry pid

Change-Id: Ic21bc29ed4daaa3cbc1f40c198bd6d4f940a4217
Reviewed-on: http://review.couchbase.org/38711
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>