if_epair: rework

Rework if_epair(4) to no longer use netisr and dpcpu.
Instead use mbufq and swi_net.
This simplifies the code and seems to make it work better and
no longer hang.

Work largely by bz@, with minor tweaks by kp@.

Reviewed by: bz, kp
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D31077

(cherry picked from commit 3dd5760aa5f876f8a3f0735afeebdf9ee414e1f5)