refactor: add most of src/util to iwyu

These files change infrequently, and not much header shuffling is required.

We don't add everything in src/util/ yet, because IWYU makes some
dubious suggestions, which I'm going to follow up with upstream.

src/util/url.cpp

@@ -5,7 +5,8 @@
 #include <util/url.h>
 
 #include <event2/http.h>
-#include <stdlib.h>
+
+#include <cstdlib>
 #include <string>
 
 std::string urlDecode(const std::string &urlEncoded) {