Decouple the UUID generator from network interfaces by having MAC

Decouple the UUID generator from network interfaces by having MAC
addresses added to the UUID generator using uuid_ether_add(). The
UUID generator keeps an arbitrary number of MAC addresses, under
the assumption that they are rarely removed (= uuid_ether_del()).
This achieves the following:
1.  It brings up closer to having the network stack as a loadable
    module.
2.  It allows the UUID generator to filter MAC addresses for best
    results (= highest chance of uniqeness).
3.  MAC addresses can come from anywhere, irrespactive of whether
    it's used for an interface or not.

A side-effect of the change is that when no MAC addresses have been
added, a random multicast MAC address is created once and re-used if
needed. Previusly, when a random MAC address was needed, it was
created for every call. Thus, a change in behaviour is introduced
for when no MAC addresses exist.

Obtained from:	Juniper Networks, Inc.