Make it possible to override default FS update algorithm

PUBLISHED_FROM=774a111548da97bd9d195e177b5fb66b0fd66135

mongoose.h

@@ -4420,8 +4420,16 @@ struct http_message {
   /* Headers */
   struct mg_str header_names[MG_MAX_HTTP_HEADERS];
   struct mg_str header_values[MG_MAX_HTTP_HEADERS];
+
+  /*
+   * Value of the Content-Length header if present,
+   * otherwise MG_HTTP_CONTENT_LENGTH_UNKNOWN.
+   */
+  size_t content_length;
 };
 
+#define MG_HTTP_CONTENT_LENGTH_UNKNOWN ((size_t) -1)
+
 #if MG_ENABLE_HTTP_WEBSOCKET
 /* WebSocket message */
 struct websocket_message {