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/environment-d-generator/environment-d-generator.c

@@ -3,7 +3,7 @@
 #include "sd-path.h"
 
 #include "conf-files.h"
-#include "def.h"
+#include "constants.h"
 #include "env-file.h"
 #include "escape.h"
 #include "glyph-util.h"