freebsd_network: Consolidate and unify SMP-related definitions.

Previously I implemented "curcpu" as ipro1000 used it, independent
of all the other SMP functions, as an optimization. Now, iflib
wants to use these functions all together, so they have to produce
consistent results.

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.