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/core/service.c

@@ -13,9 +13,9 @@
 #include "bus-kernel.h"
 #include "bus-util.h"
 #include "chase-symlinks.h"
+#include "constants.h"
 #include "dbus-service.h"
 #include "dbus-unit.h"
-#include "def.h"
 #include "env-util.h"
 #include "escape.h"
 #include "exit-status.h"