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/sysctl/sysctl.c

@@ -11,8 +11,8 @@
 
 #include "build.h"
 #include "conf-files.h"
+#include "constants.h"
 #include "creds-util.h"
-#include "def.h"
 #include "errno-util.h"
 #include "fd-util.h"
 #include "fileio.h"