spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

crypto: Add an API supporting curve25519.

This adds a wrapper around libsodium's curve25519 support matching
Linux's curve25519 API. The intended use case for this is WireGuard.

Note that this is not integrated with OCF as it is not related to
symmetric operations on data.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33935