Add raw sockets; add cluster arpwatch.

kernel/net/arp.s:
	delegate arp packets to the socket layer.

kernel/net/cloudnet.s:
	add arp watcher thread.

kernel/net/eth.s:
	make ETH_PROTO_* global.

kernel/net/socket.s:
	add support for 'raw' sockets, using address family field in flags.
	Finding sockets now filters for IP sockets;
	add new method for finding ethernet sockets (SOCK_AF_ETH) using
	ETH_PROTO values instead of IP_PROTOCOL values.