Allow SO_REUSEPORT_LB to be set on divert sockets. If set, then bind()
will add the socket to a "load-balancing group". When a divert-to rule
matches a port with an associated group, the corresponding state ID is
used to select a specific socket from the group. Packets without an
associated state are simply forwarded to the first socket in the group.
For now I only pass a state ID from pf, as I couldn't see a useful
identifier on the ipfw side.

This implementation is simple but has some caveats, the main one being
that if sockets are added to the group while flows are being processed,
the size of the group will change and this changes the mapping of state
IDs to sockets. So, to get a consistent mapping, the divert socket
application must bind all of its sockets before any traffic is processed
by pf.