General netif malloc() flags cleanup. Use M_INTWAIT or M_WAITOK instead

General netif malloc() flags cleanup.  Use M_INTWAIT or M_WAITOK instead
of M_NOWAIT.  Generally use M_WAITOK in the attach code or ioctl code
typically called from userland, and M_INTWAIT for routines that might
be called during non-boot operations.  Since M*WAIT flags guarentee a
non-NULL result, also remove now-unnecessary NULL checks.