Adding more ap_ssl_* functions and hooks to the core server.

Adding more ap_ssl_* functions and hooks to the core server.

     - ap_ssl_add_cert_files() to enable other modules like mod_md to provide
       certificate and keys for an SSL module like mod_ssl.
     - ap_ssl_add_fallback_cert_files() to enable other modules like mod_md to
       provide a fallback certificate in case no 'proper' certificate is
       available for an SSL module like mod_ssl.
     - ap_ssl_answer_challenge() to enable other modules like mod_md to
       provide a certificate as used in the RFC 8555 'tls-alpn-01' challenge
       for the ACME protocol for an SSL module like mod_ssl.
    - Hooks for 'ssl_add_cert_files', 'ssl_add_fallback_cert_files' and
      'ssl_answer_challenge' where modules like mod_md can provide providers
      to the above mentioned functions.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1887085 13f79535-47bb-0310-9956-ffa450edef68