refactor: standardize headers to #pragma once, add [[nodiscard]]/[[maybe_unused]] annotations

src/util.h

@@ -1,7 +1,8 @@
 /* Copyright and license details as described in the LICENSE file. */
+#pragma once
 
 #include <stddef.h>
 
 [[noreturn]] void die(const char *fmt, ...);
-void *ecalloc(size_t nmemb, size_t size);
-int fd_set_nonblock(int fd);
+[[nodiscard]] void *ecalloc(size_t nmemb, size_t size);
+[[nodiscard]] int fd_set_nonblock(int fd);