PPPoE: use random hunique

[Maxime Villard <max%m00nbsd.net@localhost>]

Currently, the HostUnique we send over the network is the memory address of the
active softc structure. That's dumb, as I said in TODO.kaslr.

Here is a patch [1] that fixes that: we generate a random uint64_t when a softc
structure is created, and we send this id as HostUnique.

Can someone test quickly? I know there are PPPoE users, but I don't use PPPoE
myself...

Thanks,
Maxime

[1] http://m00nbsd.net/garbage/net/pppoerand.diff