gcrypt: Return key size in bytes.

The crypto API requires get_asymmetric_key() to return the size of
the RSA key in bytes on success.  The return value is evaluated in
populate_user_list() where keys which are too short to encrypt a
challenge buffer are rejected right away.

However, for ASN.1 keys, the gcrypt implementation returned the number
of *bits* instead. This caused para_server to accept keys which are
in fact not suitable for authentication.

The fix is trivial.