sync the code with the one in stable/10
(wrap the if_t compatibilty function into a __FreeBSD_version
conditional block)
Fixes from Fanco Ficthner on transparent mode

* The way rings are updated changed with the last API bump.
  Also sync ->head when moving slots in netmap_sw_to_nic().

* Remove a crashing selrecord() call.

* Unclog the logic surrounding netmap_rxsync_from_host().

* Add timestamping to RX host ring.

* Remove a couple of obsolete comments.

Submitted by:	Franco Fichtner
MFC after:	3 days
Sponsored by:	Packetwerk