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/core/execute.c

@@ -48,10 +48,10 @@
 #include "cgroup-setup.h"
 #include "chase-symlinks.h"
 #include "chown-recursive.h"
+#include "constants.h"
 #include "cpu-set-util.h"
 #include "creds-util.h"
 #include "data-fd-util.h"
-#include "def.h"
 #include "env-file.h"
 #include "env-util.h"
 #include "errno-list.h"