mod_ssl: Log private key material to file set by $SSLKEYLOGFILE in the

mod_ssl: Log private key material to file set by $SSLKEYLOGFILE in the
environment, using the standard format which can be parsed by (e.g.)
wireshark for decoding SSL/TLS traffic; supported from OpenSSL 1.1.1.

* modules/ssl/ssl_private.h: Add keylog_file to SSLModConfigRec.

* modules/ssl/ssl_engine_init.c (ssl_init_Module): Open log file if
  SSLKEYLOGFILE is set in the environment.
  (ssl_init_ctx_protocol): Register the keylog callback with OpenSSL.

* modules/ssl/ssl_engine_kernel.c (modssl_callback_keylog):
  New function.

PR: 63391
Github: closes #74

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1869842 13f79535-47bb-0310-9956-ffa450edef68