SSL: use of SSL_MODE_NO_AUTO_CHAIN.

The SSL_MODE_NO_AUTO_CHAIN mode prevents OpenSSL from automatically
building a certificate chain on the fly if there is no certificate chain
explicitly provided.  Before this change, certificates provided via the
ssl_client_certificate and ssl_trusted_certificate directives were
used by OpenSSL to automatically build certificate chains, resulting
in unexpected (and in some cases unneeded) chains being sent to clients.

src/event/ngx_event_openssl.c

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index 83186f5..1b789e6 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -285,6 +285,10 @@
     SSL_CTX_set_mode(ssl->ctx, SSL_MODE_RELEASE_BUFFERS);
 #endif
 
+#ifdef SSL_MODE_NO_AUTO_CHAIN
+    SSL_CTX_set_mode(ssl->ctx, SSL_MODE_NO_AUTO_CHAIN);
+#endif
+
     SSL_CTX_set_read_ahead(ssl->ctx, 1);
 
     SSL_CTX_set_info_callback(ssl->ctx, ngx_ssl_info_callback);