When ALTQ was detached from an interface queue, the function pointers

When ALTQ was detached from an interface queue, the function pointers
for enqueue, dequeue and request ops where reset to NULL. That was fine
in the old days when ifq_* checked wether ALTQ is enabled and fell back
to the classic implementation, but now it just deferences a NULL pointer.
Introduce a new function ifq_set_classic, which resets the pointers to
the same values if_attach initially uses and which implements the
classic BSD queueing. Use this in altq_detach and if_attach.

Reported-by: Stefan Krüger <skrueger@meinberlikomm.de>