Add support for ssh-rsa keys.

Add support for ssh-rsa keys.

This allows to use standard ssh keys (that is, keys generated with
ssh-keygen) for the challenge/response authentication method of
paraslash. Only RSA keys without password protection are supported
at the moment.

Since we want that both openssl and ssh keys just work, we introduce
the helper function is_ssh_rsa_key(). It looks at the first few bytes
of the key to decide which type of public key we have. For openssl
keys, we just call openssl's EVP_PKEY_get1_RSA() and be done. Private
keys generated by ssh-keygen do not differ from keys generated by
"openssl rsa" and need no special treatment either.

However, public ssh rsa keys are stored differently, as an uuencoded
byte stream. So this patch adds functions that decode a given buffer
via base64 or uudecode. The two rsa public parameters (modulus and
exponent) are then read from the decoded buffer using BN_bin2bn().