Import the WireGuard driver from zx2c4.com.

This commit brings back the driver from FreeBSD commit
f187d6dfbf633665ba6740fe22742aec60ce02a2 plus subsequent fixes from
upstream.

Relative to upstream this commit includes a few other small fixes such
as additional INET and INET6 #ifdef's, #include cleanups, and updates
for recent API changes in main.

Reviewed by: pauamma, gbe, kevans, emaste
Obtained from: git@git.zx2c4.com:wireguard-freebsd @ 3cc22b2
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36909