add some definition and driver changes in preparation for

add some definition and driver changes in preparation for
two upcoming features:

semi-transparent mode:
    when a device is opened in this mode, the
    user program will be able to mark slots that must be forwarded
    to the "other" side (i.e. from NIC to host stack, or viceversa),
    and the forwarding will occur automatically at the next netmap syscall.
    This saves the need to open another file descriptor and do
    the forwarding manually.

direct-forwarding mode:
    when operating with a VALE port, the user can specify in the slot
    the actual destination port, overriding the forwarding decision
    made by a lookup of the destination MAC. This can be useful to
    implement packet dispatchers.

No API changes will be introduced.
No new functionality in this patch yet.