basic: create new basic/initrd-util.[ch] for initrd-related functions

I changed imports of util.h to initrd-util.h, or added an import of
initrd-util.h, to keep compilation working. It turns out that many files didn't
import util.h directly.

When viewing the patch, don't be confused by git rename detection logic:
a new .c file is added and two functions moved into it.

src/basic/initrd-util.h

@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <stdbool.h>
+
+bool in_initrd(void);
+void in_initrd_force(bool value);