Rename def.h to constants.h

The name "def.h" originates from before the rule of "no needless abbreviations"
was established. Let's rename the file to clarify that it contains a collection
of various semi-related constants.

src/basic/cgroup-util.h

@@ -9,7 +9,7 @@
 #include <sys/statfs.h>
 #include <sys/types.h>
 
-#include "def.h"
+#include "constants.h"
 #include "set.h"
 
 #define SYSTEMD_CGROUP_CONTROLLER_LEGACY "name=systemd"