crypto: Allow for ed25519/x25519 being available without ed448/x448

[VlkrS]

Currently, HAVE_EDDH and HAVE_EDDSA imply that both curve 25519 and curve 448 are available.

While that is the case for OpenSSL, it isn't for LibreSSL. As a consequence, OTP doesn't make TLS1.3 (which depends on HAVE_EDDH / HAVE_EDDSA) available by default when building with LibreSSL.

For OpenBSD we are currently maintaining a set of patches to remove the need for curve 448, but it would be great if we could decouple the two by introducing something like HAVE_ED25519 / HAVE_ED448.