Relax the RSA/DSA decision for SSH2 keys

[Rob-Hague]

Use the same logic as ssh-keygen:

https://github.com/openssh/openssh-portable/blob/b6b49130a0089b297245ee39e769231d7c763014/ssh-keygen.c#L510C6-L510C12

Fixes #960

(I would add a test, but I'm not sure how to generate a key with the type as in the issue)