cfg80211/nl80211: separate unicast/multicast default TX keys

Allow userspace to specify that a given key
is default only for unicast and/or multicast
transmissions. Only WEP keys are for both,
WPA/RSN keys set here are GTKs for multicast
only. For more future flexibility, allow to
specify all combiations.

Wireless extensions can only set both so use
nl80211; WEP keys (connect keys) must be set
as default for both (but 802.1X WEP is still
possible).

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>