/* call-seq:
 *   krb5.get_init_creds_keytab(principal = nil, keytab = nil, service = nil, ccache = nil)
 *
 * Acquire credentials for +principal+ from +keytab+ using +service+. If
 * no principal is specified, then a principal is derived from the service
 * name. If no service name is specified, kerberos defaults to "host".
 *
 * If no keytab file is provided, the default keytab file is used. This is
 * typically /etc/krb5.keytab.
 *
 * If +ccache+ is supplied and is a Kerberos::Krb5::CredentialsCache, the
 * resulting credentials will be stored in the credential cache.
 */
static VALUE rkrb5_get_init_creds_keytab(int argc, VALUE* argv, VALUE self){