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/locale-util.c

@@ -10,7 +10,7 @@
 #include <sys/mman.h>
 #include <sys/stat.h>
 
-#include "def.h"
+#include "constants.h"
 #include "dirent-util.h"
 #include "env-util.h"
 #include "fd-util.h"