reintroduce using the RB tree for local address lookups. this is

confusing because both addresses and broadcast addresses are put
into the tree.

there are two types of local address lookup. the first is when the
socket layer wants a local address, the second is in ip_input when
the kernel is figuring out the packet is for it to process or
forward.

ip_input considers local addresses and broadcast addresses as local,
however, the handling of broadcast addresses is different depending
on whether ip_directedbcast is set. if if ip_directbcast is unset
then a packet coming in on any interface to any of the systems
broadcast addresses is considered local, otherwise the broadcast
packet must exist on the interface it was received on.

the code also needs to consider classful broadcast addresses so we
can continue some legacy applications (eg, netbooting old sparcs
that use rarp and bootparam requests to classful broadcast addresses
as per PR6382). this diff maintains that support, but restricts it
to packets that are broadcast on the link layer (eg, ethernet
broadcasted packets), and it only looks up addresses on the local
interface. we now only support classful broadcast addresses on local
interfaces to avoid weird side effects with packets routed to us.

the ip4 socket layer does lookups for local addresses with a wrapper
around the global address tree that rejects matches against broadcast
addresses. we now no longer support bind sockets to broadcast
addresses, no matter what the value of ip_directedbcast is.

ok henning@
testing (and possibly ok) claudio@

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