freebsd_network: Clean up struct callout.

* Remove unused flags field.
* Rename internal members to have c_ at the beginning, like FreeBSD.

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.