Assume OCF is the only KTLS software backend.

This removes support for loadable software backends. The KTLS OCF
support is now always included in kernels with KERN_TLS and the
ktls_ocf.ko module has been removed. The software encryption routines
now take an mbuf directly and use the TLS mbuf as the crypto buffer
when possible.

Bump __FreeBSD_version for software backends in ports.

Reviewed by: gallatin, markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30138