new eth_tuntap.cc and eth_vde.cc

Two years later (forced to a two-days long stay in bed for a flue) I
have had time to clean up my eth_tuntap interface (it was written long
time ago as a minimal change from eth_tap).

I have also added an interface to my VDE (virtual distributed ethernet)
environment (see www.bononia.it/~renzo/keap).
VDE is useful in university computer labs to create virtual networks of
virtual machines (all at user level) also having a shared (and
controlled) Internet access. It is far less dangerous than giving users
access to /dev/net/tun.

VDE can interconnect User-Mode-Linux VMs, MPS (www.cs.unibo.it/~morsiani
the interface has not yet been released, but it is already working...), and now 
Bochs.
VDE-switches running on remote computers can be interconnected by
VDE-cables to create a single net spawned onto several physical
computers.

VDE can be connected also to tuntap interfaces: this feature can be
used:
- to interconnect the virtual net to the Internet (e.g. using NAT
  masquerated connections)
- real physical linux box can join the virtual net
- VDE can be used as a general tunneling tool (e.g. to test IPv6 on a
  IPv4 only infrastructure). There is also the support for mobility
  (vde-cables can be changed run time keeping the entire infrastruture
  running. e.g. real IP addresses can be changed but the virtual net is
  not affected by the change. During hand-offs there can be even several
  cables in place to prevent hitchups in connectivity).

I hope you like this new development.

ciao from Italy.
                        renzo

eth_tuntap.cc
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eth_vde.cc
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eth.cc
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