Changes

Refactor tun_t fields and alloc APIs

The previous state of code made allocation code more complex for no good
reason; use usual alloc() type function instead, splitting between the 2
types of tun_t implementations available to make it easier to
understand.

Move the several tun-iface specific mode fields to a substruct to make
it clear those are only really containing useful information when the
tun_t is created in tun-iface mode.

Change-Id: Ic71f91c62cd5bd48c6d35534eaac2091e4c69735