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/unit-file.c

@@ -6,6 +6,7 @@
 #include "dirent-util.h"
 #include "fd-util.h"
 #include "fs-util.h"
+#include "initrd-util.h"
 #include "macro.h"
 #include "path-lookup.h"
 #include "set.h"