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/hostname/hostnamed.c

@@ -11,7 +11,7 @@
 #include "bus-get-properties.h"
 #include "bus-log-control-api.h"
 #include "bus-polkit.h"
-#include "def.h"
+#include "constants.h"
 #include "env-file-label.h"
 #include "env-file.h"
 #include "env-util.h"