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

(cherry picked from commit 0c6274a819ffdf6d5a3713b2c0f7014840f01703)