curve: rename encode/decode buffers

No need to prefix them with _buf, this just makes the names longer
and therefore less readable.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
curve: simplify curve25519_proto_init and others

Simplify the function curve25519_proto_init() and also have minor
cleanups in other related ones.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
curve: curve25519_encode/curve25519_decode: simplify crypto transforms

Simplify curve25519_encode() and curve25519_decode() transforms by making
the code more clear and by introducing macro accessors to nonces.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>