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/debug-generator/debug-generator.c

@@ -5,6 +5,7 @@
 #include "alloc-util.h"
 #include "dropin.h"
 #include "generator.h"
+#include "initrd-util.h"
 #include "mkdir-label.h"
 #include "parse-util.h"
 #include "path-util.h"