nl80211: Add generic netlink module alias for cfg80211/nl80211

To support auto-loading of wireless modules from netlink users, add module
alias for nl80211 family.

This also adds NL80211_GENL_NAME constant to define the "nl80211" netlink
family name as part of uapi.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>