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/debug-generator/debug-generator.c

@@ -5,7 +5,7 @@
 #include "alloc-util.h"
 #include "dropin.h"
 #include "generator.h"
-#include "mkdir.h"
+#include "mkdir-label.h"
 #include "parse-util.h"
 #include "path-util.h"
 #include "proc-cmdline.h"
@@ -138,7 +138,7 @@ static int generate_wants_symlinks(void) {
                 if (!f)
                         return log_oom();
 
-                mkdir_parents_label(p, 0755);
+                (void) mkdir_parents_label(p, 0755);
 
                 if (symlink(f, p) < 0)
                         r = log_error_errno(errno,