Add libcrypto to the the list of dependent libraries under FreeBSD.
- Remove unnecessary initial syncs in the if_netmap transmit and
  receive threads.

- Remove unnecessary rx sync in the if_netmap receive thread.

- Handle EBUSY return from tx sync (which represents concurrent access
  to the same ring by another actor, such as the vale switch) and only
  perform the sync when not otherwise performed by poll.