Move portions of data structure initialization from if_attach() to

Move portions of data structure initialization from if_attach() to
if_alloc(), and portions of data structure destruction from if_detach()
to if_free().  These changes leave more of the struct ifnet in a
safe-to-access condition between alloc and attach, and between detach
and free, and focus on attach/detach as stack usage events rather than
data structure initialization.

Affected fields include the linkstate task queue, if_afdata lock,
address lists, kqueue state, and MAC labels.  ifq_attach() ifq_detach()
are not moved as ifq_attach() may use a queue length set by the device
driver between if_alloc() and if_attach().

MFC after:	3 weeks