Rewrite tun/tap driver using better C

[rlupton20]

The tun/tap driver in Network/ is essentially a verbatim copy of the example linux tun/tap driver. Buffer safety is dealt with in (some not so readable) Haskell.

The C interface should be written to be
a) more defensive
b) multithreadable (investigate - if possible)
c) more configurable

the Haskell wrapper might provide more options and flags. Continuation passing style seems a good way to expose this to other Haskell code (withTUN is an example of this).