[mod_openssl] ssl.openssl.ssl-conf-cmd (fixes #2758)

[mod_openssl] ssl.openssl.ssl-conf-cmd (fixes #2758)

(similar to Apache mod_ssl SSLOpenSSLConfCmd directive)

(experimental)

This new directive is for use with OpenSSL only, and is not currently
available in LibreSSL.

https://wiki.openssl.org/index.php/Manual:SSL_CONF_cmd(3)

lighttpd takes "file commands" not "command line commands" as
openssl SSL_CONF_cmd() appears to permit only one mode at a time.

lighttpd processes this directive after all other ssl.* directives
have been applied for the $SERVER["socket"] scope.

x-ref:
  "Option to disable TLS session tickets"
  https://redmine.lighttpd.net/issues/2758
  "Allow to selectively disable TLS 1.0, 1.1 and 1.2 versions"
  https://github.com/lighttpd/lighttpd1.4/pull/84

github: closes #84