Use void arguments list consistently in C code

On my way, fixed a couple of cases where we had `()` in the header, and
non-empty argument list in the source file.

PUBLISHED_FROM=5519526cf84e2bbd425a726fcc112fea1a95cbf1

examples/simple_crawler/simple_crawler.c

@@ -40,7 +40,7 @@ static void event_handler(struct mg_connection *nc, int event, void *data) {
   }
 }
 
-int main() {
+int main(void) {
   struct mg_mgr mgr;
 
   mg_mgr_init(&mgr, NULL);