netdev-linux: Cleanup tap netdev.

netdev-linux: Cleanup tap netdev.

TAP devices need to be treated slightly differently from other other
devices because they cannot be opened multiple times.  Instead we
open them once and share the file descriptor.  This means that if
the netdev is opened multiple times one reader can drain the buffers
of another.  While this is a deviation from the normal convention,
it does not impact current or planned users.

In addition, this cleans up some confusion between the file
descriptor for tap devices versus other FD's.