It is replaced by kernel threads that do the same thing. The number of kernel threads used is set with the vfs.nfs.iothreads sysctl.
This replaces the previous /etc/rc.d/sysctl. Also, the variables are now set earlier, between ipf/ipnat and the network coming up. The rc.conf defcorename and securelevel variables are no more. You can set them directly in sysctl.conf now.