nl80211: Resubscribe to nl80211 events on global nl_event socket

nl80211: Resubscribe to nl80211 events on global nl_event socket

This allows wpa_supplicant to recover from some of the cases where
cfg80211 is unloaded and reloaded without restarting wpa_supplicant. The
netlink socket used for nl80211 events (global->nl_event) seemed to end
up in otherwise functionality state, but with all the event memberships
lost when cfg80211 gets reloaded.

There does not seem to be any clear way of determining when this has
happened, so it looks simplest to just try to re-subscribe to all the
events whenever an interface is re-enabled or added.

Signed-off-by: Jouni Malinen <j@w1.fi>