Adds a public key from the X.509 certificate to the key ring.
Index of the newly added public key.
Use this method when you need to add new public key from the certificate to key ring.
If the certificate contains secret key it will be added as well.
E-mail address from the certificate will be used as UserID. If there is no e-mail address than commonName is used,
if there is no commonName also organizationName will be assigned to UserID.
If such key is already present in the keyring only new information will be added.