/*
 * call-seq:
 *   krb5.get_init_creds_password(user, password, service = nil)
 *
 * Authenticates the credentials of +user+ using +password+ against +service+,
 * and has the effect of setting the principal and context internally. This method
 * must typically be called before using other methods.
 */
static VALUE rkrb5_get_init_creds_passwd(int argc, VALUE* argv, VALUE self){