new ng_device

[Gleb Smirnoff]

 Here is a major rewrite of ng_device. The main
differencies with current one are:

- one dev per node
- locking
- read queue implemented using struct ifqueue, thus
  reducing number of reads/writes to temporary buffers.

A more detailed list of differencies is attached. A test program
also attached.

P.S. Yet another crazy netgraph idea: ng_iface + ng_device is
a substitute for tun(4). I should write a patch for ppp(8)
to work with ng_iface. :)

-- 
Totus tuus, Glebius.
GLEBIUS-RIPN GLEB-RIPE