shared: clean up mkdir.h/label.h situation

Previously the mkdir_label() family of calls was implemented in
src/shared/mkdir-label.c but its functions partly declared ins
src/shared/label.h and partly in src/basic/mkdir.h (!!). That's weird
(and wrong).

Let's clean this up, and add a proper mkdir-label.h matching the .c
file.

src/shared/dev-setup.c

@@ -8,6 +8,7 @@
 #include "dev-setup.h"
 #include "label.h"
 #include "log.h"
+#include "mkdir-label.h"
 #include "nulstr-util.h"
 #include "path-util.h"
 #include "umask-util.h"