Make it possible to override default FS update algorithm

PUBLISHED_FROM=774a111548da97bd9d195e177b5fb66b0fd66135

docs/c-api/mg_http.h/struct_http_message.md

@@ -29,6 +29,12 @@ signature: |
     /* 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;
   };
 ---