Initial support for DSA private keys.

[adiroiban]

Scope

This adds support for loading DSA private keys in legacy OpenSSH PEM format.

I was not able to find the RFC in which the format is defined.

I named it DSAPrivateKey and not OpenSSHDSAPrivateKey as you can also generate it using OpenSSL.

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[hynek]

Thanks! I'll tweak the changelog a bit but I didn't want to do a review round around it.