neuter the tun/tap ioctls that try and modify interface flags.

neuter the tun/tap ioctls that try and modify interface flags.

historically there was just tun(4) that supported both layer 3 p2p
and ethernet modes, but had to be reconfigured at runtime by userland
to properly change the interface type and interface flags. this is
obviously not a great idea, mostly because a lot of stack behaviour
around address management makes assumptions based on these parameters,
and changing them at runtime confuses things.

splitting tun so ethernet was handled by a specific tap(4) driver
was a first step at locking this down. this takes a further step
by restricting userlands ability to reconfigure the interface flags,
specifically IFF_BROADCAST, IFF_MULTICAST, and IFF_POINTOPOINT.

this change lets userland pass those values via the ioctls, but
only if they match the current set of flags on the interface. these
flags are set appropriate for the type of interface when it's
created, but should not be changed afterwards.

nothing in base uses these ioctls, so the only fall out will be
from ports doing weird things.

ok claudio@ kn@