Add some improvements as suggested by Kaspar

- expand comment in config file
- check username == NULL
- detect SRP support via SSL_CTRL_SET_TLS_EXT_SRP_USERNAME_CB, not via openssl
  version
- rename rv variable


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1348653 13f79535-47bb-0310-9956-ffa450edef68