hostapd: provide BSS-transition-queries to ubus subscribers

hostapd: provide BSS-transition-queries to ubus subscribers

Provide incoming BSS transition queries to ubus subscribers.

This allows external steering daemons to provide clients with
an optimal list of transition candidates.

This commit has no functional state in case no ubus subscriber is
present or it does not handle this ubus message.

To prevent hostapd from sending out a generic response by itself, a
subscribing daemon has to return a non-zero response code to hostapd.

Signed-off-by: David Bauer <mail@david-bauer.net>