Introduce new function if_unit(9). This function returns a pointer the

Introduce new function if_unit(9). This function returns a pointer the
interface descriptor corresponding to the unique name. This descriptor
is guaranteed to be valid until if_put(9) is called on the returned
pointer. if_unit(9) should replace already existent ifunit() which
returns descriptor not safe for dereference when context was switched.
This allow us to avoid some use-after-free issues in ioctl(2) path.
Also this unifies interface descriptor usage.

ok claudio@ sashan@