Use set_key addr to distinguish default and multicast keys

Previously, both NULL and ff:ff:ff:ff:ff:ff addr were used in various
places to indicate default/broadcast keys. Make this more consistent
and useful by defining NULL to mean default key (i.e., used both for
unicast and broadcast) and ff:ff:ff:ff:ff:ff to indicate broadcast
key (i.e., used only with broadcast).

src/utils/common.h

@@ -465,6 +465,8 @@ static inline int is_broadcast_ether_addr(const u8 *a)
 	return (a[0] & a[1] & a[2] & a[3] & a[4] & a[5]) == 0xff;
 }
 
+#define broadcast_ether_addr (const u8 *) "\xff\xff\xff\xff\xff\xff"
+
 #include "wpa_debug.h"