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/procfs-util.c

@@ -4,7 +4,7 @@
 #include <unistd.h>
 
 #include "alloc-util.h"
-#include "def.h"
+#include "constants.h"
 #include "fd-util.h"
 #include "fileio.h"
 #include "parse-util.h"