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/syserror.cpp

@@ -10,6 +10,7 @@
 #include <util/syserror.h>
 
 #include <cstring>
+#include <string>
 
 std::string SysErrorString(int err)
 {