Address https://github.com/cesanta/mongoose/discussions/2305 - bind to v6 only / v6 and v4

src/config.h

@@ -44,6 +44,10 @@
 #define MG_ENABLE_IPV6 0
 #endif
 
+#ifndef MG_IPV6_V6ONLY
+#define MG_IPV6_V6ONLY 0  // IPv6 socket binds only to V6, not V4 address
+#endif
+
 #ifndef MG_ENABLE_MD5
 #define MG_ENABLE_MD5 1
 #endif