freebsd_network: Move ticks/seconds conversions to sys/time.h and
rename.

They now match the names they have in FreeBSD.
No functional change intended.

freebsd_network: Lots of additions and modifications for iflib.

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.