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/machine/machinectl.c

@@ -25,8 +25,8 @@
 #include "bus-wait-for-jobs.h"
 #include "cgroup-show.h"
 #include "cgroup-util.h"
+#include "constants.h"
 #include "copy.h"
-#include "def.h"
 #include "env-util.h"
 #include "fd-util.h"
 #include "format-table.h"