Modularizing the network stack with a driver API

Hi everyone,

Back in 2014 Marcel Moolenaar started the thread "Roadmap for ifnet(9)
for FreeBSD 11"
(https://lists.freebsd.org/pipermail/freebsd-arch/2014-May/015379.html),
and after 8 years we want to revisit this.  This email is to kick
things off again, and further design discussions.

I've spent time off and on as able over the last couple years porting
-CURRENT to this "DrvAPI", and have something that compiles.  Much of
the work was committed at the time of the initial discussion in 2014,
with enhancements done off and on since then.  Also, much of the
shortcomings listed at https://wiki.freebsd.org/projects/ifnet have not
been addressed at all yet.

Most of the work I've done in the recent port has been purely
mechanical and scripted, fixing build failures along the way.  The
current work in progress can be found in my personal repository at
https://github.com/chmeeedalf/freebsd/tree/drvapi .  The goal of this
first step is to get things started, address design feedback, and move
forward in main.



- Justin