spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

chacha20_poly1305: Use the correct license disclaimer.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33976

crypto: Add a simple API for [X]ChaCha20-Poly1035 on flat buffers.

This is a synchronous software API which wraps the existing software
implementation shared with OCF. Note that this will not currently
use optimized backends (such as ossl(4)) but may be appropriate for
operations on small buffers.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33524