Fix a possible memory leak in libwebsocket_context_destroy() when compiled without extension support.

Even without extension support :
  All wsi sessions stored into lws_lookup[] SHOULD BE closed_and_freed.
  The callback(reason=_PROTOCOL_DESTROY) SHOULD BE called.
Place lws_sigusr2() inside #ifdef LWS_OPENSSL_SUPPORT flags
because when compiling without SSL, 'gcc -Werror' command fails saying lws_sigusr2 is defined but not used