mod_ssl: Add support for loading private keys from ENGINEs. Support

for PKCS#11 URIs only, and PIN entry is not threaded through
SSLPassPhraseDialog config yet.

* modules/ssl/ssl_util.c (modssl_is_engine_key): New function.

* modules/ssl/ssl_engine_config.c (ssl_cmd_SSLCertificateKeyFile):
  Use it, skip check for file existence for engine keys.

* modules/ssl/ssl_engine_pphrase.c (modssl_load_engine_pkey):
  New function.

* modules/ssl/ssl_engine_init.c (ssl_init_server_certs):
  For engine keys, load via modssl_load_engine_pkey.

Submitted by: Anderson Sasaki <ansasaki redhat.com>, jorton


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1830819 13f79535-47bb-0310-9956-ffa450edef68