tls: openssl: add runtime enable/disable API for SSL key logging

[VaibhavTekale1]

@lws-team

Summary

Add two new public APIs to enable and disable SSL key logging (SSLKEYLOGFILE)
at runtime, without requiring a context rebuild.

Previously, the keylog callback was only registered if keylog_file was set
at context creation time (lws_context_creation_info). This made it impossible
to toggle key logging on an already-running context.

Changes

New public APIs (include/libwebsockets/lws-context-vhost.h)

• lws_set_keylog_file(struct lws *wsi, char *sslkeyfilepath)
  Enables SSL key logging. If sslkeyfilepath is non-empty, that path is used;
  otherwise the SSLKEYLOGFILE environment variable is consulted. The resolved
  path is written into wsi->a.context->keylog_file.

• lws_reset_keylog_file(struct lws *wsi)
  Disables SSL key logging by clearing wsi->a.context->keylog_file.

TLS callback registration (lib/tls/openssl/)

• openssl-client.c, openssl-server.c: SSL_CTX_set_keylog_callback() is
  now registered unconditionally (subject to LWS_HAVE_SSL_CTX_set_keylog_callback
  and LWS_WITH_TLS). The lws_klog_dump callback itself gates on whether
  keylog_file is set, so logging only occurs when explicitly enabled via the
  new APIs.

• Simplified the preprocessor condition in openssl-server.c from
  (!defined(LWS_WITHOUT_CLIENT) || !defined(LWS_WITHOUT_SERVER)) to just
  defined(LWS_WITH_TLS), which is the meaningful guard for this code path.

[lws-team]

0. This looks very wrong, a long env var value will buffer overflow keylog_file

   if (NULL != klfl_env && strlen(klfl_env) > 1){
   lws_strncpy(wsi->a.context->keylog_file, klfl_env,
   strlen(klfl_env)+1);
   }

1. Why don't you just have one api, and set the filepath to NULL to do what you "reset" api does?

2. Why do the apis you propose take a wsi and operate on the context? You should take a pointer to the context?

3. Why do the apis you propose operate on the context anyway? The app may have many vhosts, shouldn't we be trying to turn this on and off per vhost?

4. Why did you change this? It shouldn't break anything to have it and may break things to remove it

• #if defined(LWS_HAVE_SSL_CTX_set_keylog_callback) && \

•   defined(LWS_WITH_TLS) && (!defined(LWS_WITHOUT_CLIENT) || !defined(LWS_WITHOUT_SERVER))
  

• #if defined(LWS_HAVE_SSL_CTX_set_keylog_callback) && defined(LWS_WITH_TLS)