mod_ssl: Add support for loading TLS certificates through the PKCS#11

engine.

* modules/ssl/ssl_util.c (modssl_is_engine_id): Renamed
  from modssl_is_engine_key.
  
* modules/ssl/ssl_engine_config.c (ssl_cmd_SSLCertificateKeyFile):
  Adjust accordingly.
  (ssl_cmd_SSLCertificateFile): Also allow ENGINE cert ids.

* modules/ssl/ssl_engine_pphrase.c (modssl_load_engine_keypair):
  Rename from modssl_load_engine_key; load certificate if
  cert id is passed.

* modules/ssl/ssl_engine_init.c (ssl_init_server_certs): Optionally
  load the certificate from the engine as well.

* docs/manual/: Update manual.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1831168 13f79535-47bb-0310-9956-ffa450edef68