*) mod_md: bringing over v2.0.6 from github.

  *) mod_md: bringing over v2.0.6 from github.
     - supports the ACMEv2 protocol
     - supports the new challenge method 'tls-alpn-01'
     - supports command configuration to setup/teardown 'dns-01' challenges
     - supports wildcard certificates when dns challenges are configured
     - ACMEv2 is the new default and will be used on the next certificate renewal,
       unless another MDCertificateAuthority is configured
     - challenge type 'tls-sni-01' has been removed as CAs do not offer this any longer
     - a domain exposes its status at https://<domain>/.httpd/certificate-status
     - Managed Domains are now in Apache's 'server-status' page
     - A new handler 'md-status' exposes verbose status information in JSON format
     - new directives "MDCertificateFile" and "MDCertificateKeyFile" to configure a
       Managed Domain that uses static files. Auto-renewal is turned off for those.
     - new MDMessageCmd that is invoked on several events: 'renewed', 'expiring' and
       'errored'. New 'MDWarnWindow' directive to configure when expiration warnings
       shall be issued.
     - ACMEv2 endpoints use the GET via empty POST way of accessing resources, see
       announcement by Let's Encrypt:
       https://community.letsencrypt.org/t/acme-v2-scheduled-deprecation-of-unauthenticated-resource-gets/74380

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1862013 13f79535-47bb-0310-9956-ffa450edef68