ubus: add ACL support for "subscribe" request

ubus: add ACL support for "subscribe" request

With this change ubus will allow users with access to the object pseudo
method ":subscribe" to subscribe for notifications.

1. Move uh_ubus_allowed() up in the code
2. Export "Authorization" parsing code to the uh_ubus_get_auth()
3. Check for ":subscribe" method access

Right now this depends on "Authorization" HTTP header which browsers
don't allow setting for the EventSource. An alternative method of
submitting session token remains to be implemented.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>