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/initctl/initctl.c

@@ -14,8 +14,8 @@
 #include "alloc-util.h"
 #include "bus-error.h"
 #include "bus-util.h"
+#include "constants.h"
 #include "daemon-util.h"
-#include "def.h"
 #include "fd-util.h"
 #include "format-util.h"
 #include "initreq.h"