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If I try to generate an ed25519 key, it is *not* an ECDSA key.  But I
see this warning:

    0 dkg@host:~$ certtool --generate-privkey --provable --key-type ed25519
    Generating a 256 bit EdDSA (Ed25519) private key ...
    The --provable parameter cannot be used with ECDSA keys.
    1 dkg@host:~$

Looking at the code and documentation, it's clear that --provable only
works for RSA and DSA.  This fix aligns the warning message with the
underlying mechanism.
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>