nl80211: support vendor-specific events

In addition to vendor-specific commands, also support vendor-specific
events. These must be registered with cfg80211 before they can be used.
They're also advertised in nl80211 in the wiphy information so that
userspace knows can be expected. The events themselves are sent on a
new multicast group called "vendor".

Signed-off-by: Johannes Berg <johannes.berg@intel.com>