cfg80211: add add_nan_func / del_nan_func

A NAN function can be either publish, subscribe or follow
up. Make all the necessary verifications and just pass the
request to the driver.
Allow the user space application that starts NAN to
forbid any other socket to add or remove functions.

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Ayala Beker <ayala.beker@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>