freebsd_network: Adaptations in preparation for the OpenBSD layer.

Most of these changes however add things that FreeBSD has (with a few
exceptions noted in comments) which the OpenBSD shim layer merely needs
to make use of.

(FreeBSD used to have support for IFF_NEEDSGIANT but removed it. The
support instated here is very similar to what FreeBSD used to have.)

freebsd11_network -> freebsd_network.

FreeBSD 12 has no major changes to the ifnet KPIs that constitute a
source compatibility break, save a single one related to locking
which doesn't really apply to us, and so we don't need to create
a "freebsd12_network" directory to work through the upgrades.