BUG/MEDIUM: ssl: ECDHE ciphers not usable without named curve configured.

Fix consists to use prime256v1 as default named curve to init ECDHE ciphers if none configured.
diff --git a/include/common/defaults.h b/include/common/defaults.h
index 9b54276..a247faf 100644
--- a/include/common/defaults.h
+++ b/include/common/defaults.h
@@ -191,6 +191,11 @@
 #define LISTEN_DEFAULT_CIPHERS NULL
 #endif
 
+/* named curve used as defaults for ECDHE ciphers */
+#ifndef ECDHE_DEFAULT_CURVE
+#define ECDHE_DEFAULT_CURVE "prime256v1"
+#endif
+
 /* ssl cache size */
 #ifndef SSLCACHESIZE
 #define SSLCACHESIZE 20000