Add "priv:" tag to private key files

Raw representation of ed25519 keys look the same for private and public keys, as strings of 64 hexadecimal digits, which means that type of key must be known from context.

If we add a prefix, e.g., "priv:" in private key files, we remove ambiguity. Tools that expect a public key can then reject a private key passed by mistake. In addition, tools that expect a private key can accept a public key, and then automatically use ssh-agent for private key operations (currently, this works, but only with public keys in ssh format).