things a bit:
- use dpcpu data to track the ifps with packets queued up,
- per-cpu locking and driver flags
- along with .nh_drainedcpu and NETISR_POLICY_CPU.
- Put the mbufs in flight reference count, preventing interfaces
from going away, under INVARIANTS as this is a general problem
of the stack and should be solved in if.c/netisr but still good
to verify the internal queuing logic.
- Permit changing the MTU to virtually everythinkg like we do for loopback.
Hook epair(4) up to the build.
Approved by: re (kib)
