- Expand scope of tun/tap softc locks to cover more softc fields and

- Expand scope of tun/tap softc locks to cover more softc fields and
  driver-maintained ifnet fields (such as if_drv_flags).
- Use soft locks as the mutex that protects each interface's knote list
  rather than using the global knote list lock.  Also, use the softc
  for kn_hook instead of the cdev.
- Use mtx_sleep() instead of tsleep() when blocking in the read routines.
  This fixes a lost wakeup race.
- Remove D_NEEDGIANT now that the cdevsw routines use the softc lock
  where locking is needed.
- Lock IFQ when calculating the result for FIONREAD in tap(4).  tun(4)
  already did this.
- Remove remaining spl calls.

Submitted by:	Marcin Cieslak  saper of saper|info (3)
MFC after:	2 weeks