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
The idea is untouched upstream sources live in sys/contrib/libsodium.
sys/crypto/libsodium are support routines or compatibility headers to allow
building unmodified upstream code.
This is not yet integrated into the build system, so no functional change.
