Move GeneralName parsing code to ipalib.x509
    
    GeneralName parsing code is primarily relevant to X.509.  An
    upcoming change will add SAN parsing to the cert-show command, so
    first move the GeneralName parsing code from ipalib.pkcs10 to
    ipalib.x509.
    
    Part of: https://fedorahosted.org/freeipa/ticket/6022
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>