More finishing touches for SSLOpenSSLConfCmd:

- add documentation
- clear the error queue before executing the next SSL_CONF_cmd
- if needed, configure OCSP stapling after a "Certificate" command
- ifdef ssl_cmd_SSLOpenSSLConfCmd in ssl_private.h


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1555464 13f79535-47bb-0310-9956-ffa450edef68