ecc: Add OIDs from RFC-4880 as aliases for Ed25519 and Curve25519.

This also adds X25519 as an alias.

Note that we only support the PureEdDSA algorithms and not
HashEdDSA (i.e. Ed25519ph)

We have a similar change in 1.9.

• Signed-off-by: Werner Koch <wk@gnupg.org>